diff --git a/src/hydro/Gizmo/hydro.h b/src/hydro/Gizmo/hydro.h
index f76d1a10e91eb07e8e0aaf1ac19a4fa17b7959fb..a7ccde6a9bcd2ea87e16b89e07bcb4782e7044ad 100644
--- a/src/hydro/Gizmo/hydro.h
+++ b/src/hydro/Gizmo/hydro.h
@@ -17,6 +17,8 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  ******************************************************************************/
+#ifndef SWIFT_GIZMO_HYDRO_H
+#define SWIFT_GIZMO_HYDRO_H
 
 #include "adiabatic_index.h"
 #include "approx_math.h"
@@ -840,3 +842,5 @@ __attribute__((always_inline)) INLINE static void hydro_set_entropy(
 #endif
   p->primitives.P = S * pow_gamma(p->primitives.rho);
 }
+
+#endif /* SWIFT_GIZMO_HYDRO_H */
diff --git a/src/hydro/Gizmo/hydro_debug.h b/src/hydro/Gizmo/hydro_debug.h
index f11104f8a6847300e47ffd6618acc4498bb5e648..0516068d3452e179c076f40a7c2db859be1c73c6 100644
--- a/src/hydro/Gizmo/hydro_debug.h
+++ b/src/hydro/Gizmo/hydro_debug.h
@@ -16,6 +16,8 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  ******************************************************************************/
+#ifndef SWIFT_GIZMO_HYDRO_DEBUG_H
+#define SWIFT_GIZMO_HYDRO_DEBUG_H
 
 __attribute__((always_inline)) INLINE static void hydro_debug_particle(
     const struct part* p, const struct xpart* xp) {
@@ -75,3 +77,5 @@ __attribute__((always_inline)) INLINE static void hydro_debug_particle(
       p->geometry.matrix_E[2][1], p->geometry.matrix_E[2][2],
       p->timestepvars.vmax, p->density.wcount_dh, p->density.wcount);
 }
+
+#endif /* SWIFT_GIZMO_HYDRO_DEBUG_H */
diff --git a/src/hydro/Gizmo/hydro_gradients.h b/src/hydro/Gizmo/hydro_gradients.h
index b01b4d90b845520c2bd20b4d33279b5d533999d2..8513308cc9c5de6bac489e79786fddc8af7e4258 100644
--- a/src/hydro/Gizmo/hydro_gradients.h
+++ b/src/hydro/Gizmo/hydro_gradients.h
@@ -1,4 +1,3 @@
-
 /*******************************************************************************
  * This file is part of SWIFT.
  * Copyright (c) 2016 Bert Vandenbroucke (bert.vandenbroucke@gmail.com)
@@ -18,8 +17,8 @@
  *
  ******************************************************************************/
 
-#ifndef SWIFT_HYDRO_GRADIENTS_H
-#define SWIFT_HYDRO_GRADIENTS_H
+#ifndef SWIFT_HYDRO_GIZMO_GRADIENTS_H
+#define SWIFT_HYDRO_GIZMO_GRADIENTS_H
 
 #include "hydro_slope_limiters.h"
 #include "hydro_unphysical.h"
@@ -160,4 +159,4 @@ __attribute__((always_inline)) INLINE static void hydro_gradients_predict(
   gizmo_check_physical_quantity("pressure", Wj[4]);
 }
 
-#endif  // SWIFT_HYDRO_GRADIENTS_H
+#endif  /* SWIFT_HYDRO_GIZMO_GRADIENTS_H */
diff --git a/src/hydro/Gizmo/hydro_gradients_gizmo.h b/src/hydro/Gizmo/hydro_gradients_gizmo.h
index bc50c10d84cdd6b444887a8bb5fdf7b49a004eb8..034f03c619a2815912a163f5ba7487cf0761dbbe 100644
--- a/src/hydro/Gizmo/hydro_gradients_gizmo.h
+++ b/src/hydro/Gizmo/hydro_gradients_gizmo.h
@@ -22,6 +22,9 @@
  *
  * @param p Particle.
  */
+#ifndef SWIFT_GIZMO_HYDRO_GRADIENTS_H
+#define SWIFT_GIZMO_HYDRO_GRADIENTS_H
+
 __attribute__((always_inline)) INLINE static void hydro_gradients_init(
     struct part *p) {
 
@@ -484,3 +487,5 @@ __attribute__((always_inline)) INLINE static void hydro_gradients_finalize(
 
   hydro_slope_limit_cell(p);
 }
+
+#endif /* SWIFT_GIZMO_HYDRO_GRADIENTS_H */
diff --git a/src/hydro/Gizmo/hydro_gradients_sph.h b/src/hydro/Gizmo/hydro_gradients_sph.h
index f635faecea549f7da280ade9b944021a5e4aeb4c..56a5a88cc184d0260036c8557aacf7a3f198213f 100644
--- a/src/hydro/Gizmo/hydro_gradients_sph.h
+++ b/src/hydro/Gizmo/hydro_gradients_sph.h
@@ -22,6 +22,9 @@
  *
  * @param p Particle.
  */
+#ifndef SWIFT_GIZMO_HYDRO_SPH_GRADIENTS_H
+#define SWIFT_GIZMO_HYDRO_SPH_GRADIENTS_H
+
 __attribute__((always_inline)) INLINE static void hydro_gradients_init(
     struct part *p) {
 
@@ -246,3 +249,5 @@ __attribute__((always_inline)) INLINE static void hydro_gradients_finalize(
 
   hydro_slope_limit_cell(p);
 }
+
+#endif /* SWIFT_GIZMO_HYDRO_SPH_GRADIENTS_H */
diff --git a/src/hydro/Gizmo/hydro_iact.h b/src/hydro/Gizmo/hydro_iact.h
index c2ed5dfd6fd4f137f44b35887b299d3c3ba8337a..a42c3d6a77c5d25e32c5f34cc581d8cbec1ffba9 100644
--- a/src/hydro/Gizmo/hydro_iact.h
+++ b/src/hydro/Gizmo/hydro_iact.h
@@ -18,6 +18,8 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  ******************************************************************************/
+#ifndef SWIFT_GIZMO_HYDRO_IACT_H
+#define SWIFT_GIZMO_HYDRO_IACT_H
 
 #include "adiabatic_index.h"
 #include "hydro_gradients.h"
@@ -486,3 +488,5 @@ __attribute__((always_inline)) INLINE static void runner_iact_nonsym_force(
 
   runner_iact_fluxes_common(r2, dx, hi, hj, pi, pj, 0);
 }
+
+#endif /* SWIFT_GIZMO_HYDRO_IACT_H */
diff --git a/src/hydro/Gizmo/hydro_io.h b/src/hydro/Gizmo/hydro_io.h
index 72f75405ded2f2a9123d586ee606c07e35deb6ec..aa26316d54c003c2fbd1fda7d05eadfb222c5fc2 100644
--- a/src/hydro/Gizmo/hydro_io.h
+++ b/src/hydro/Gizmo/hydro_io.h
@@ -16,6 +16,8 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  ******************************************************************************/
+#ifndef SWIFT_GIZMO_HYDRO_IO_H
+#define SWIFT_GIZMO_HYDRO_IO_H
 
 #include "adiabatic_index.h"
 #include "hydro.h"
@@ -186,3 +188,5 @@ void hydro_write_flavour(hid_t h_grpsph) {
  * @return 1 if entropy is in 'internal energy', 0 otherwise.
  */
 int writeEntropyFlag() { return 0; }
+
+#endif /* SWIFT_GIZMO_HYDRO_IO_H */
diff --git a/src/hydro/Gizmo/hydro_slope_limiters.h b/src/hydro/Gizmo/hydro_slope_limiters.h
index cd66f05ac9eb9d51744723d93f899b0c8c668e2e..acf3364a054ff66933c5a8c674abf820fb519913 100644
--- a/src/hydro/Gizmo/hydro_slope_limiters.h
+++ b/src/hydro/Gizmo/hydro_slope_limiters.h
@@ -91,4 +91,4 @@ __attribute__((always_inline)) INLINE static void hydro_slope_limit_cell(
 
 #endif
 
-#endif  // SWIFT_HYDRO_SLOPE_LIMITERS_H
+#endif  /* SWIFT_HYDRO_SLOPE_LIMITERS_H */
diff --git a/src/hydro/Gizmo/hydro_slope_limiters_cell.h b/src/hydro/Gizmo/hydro_slope_limiters_cell.h
index aa99b43721f669f47a7888a5da0b1933ca1ebd62..ccab0a5d00e8c13a2529efb55e49c87360a436ef 100644
--- a/src/hydro/Gizmo/hydro_slope_limiters_cell.h
+++ b/src/hydro/Gizmo/hydro_slope_limiters_cell.h
@@ -16,6 +16,8 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  ******************************************************************************/
+#ifndef SWIFT_GIZMO_SLOPE_LIMITER_CELL_H
+#define SWIFT_GIZMO_SLOPE_LIMITER_CELL_H
 
 #include <float.h>
 
@@ -171,3 +173,5 @@ __attribute__((always_inline)) INLINE static void hydro_slope_limit_cell(
     p->primitives.gradients.P[2] *= alpha;
   }
 }
+
+#endif /* SWIFT_GIZMO_SLOPE_LIMITER_CELL_H */
diff --git a/src/hydro/Gizmo/hydro_slope_limiters_face.h b/src/hydro/Gizmo/hydro_slope_limiters_face.h
index ba96063d661a93a4efc4069ff7e7269a4ac58c3b..0d96dc4d434213008bd56104674c9bc6c60437b5 100644
--- a/src/hydro/Gizmo/hydro_slope_limiters_face.h
+++ b/src/hydro/Gizmo/hydro_slope_limiters_face.h
@@ -29,6 +29,9 @@
  * @return The slope limited difference between the quantity at the particle
  * position and the quantity at the interface position.
  */
+#ifndef SWIFT_GIZMO_SLOPE_LIMITER_FACE_H
+#define SWIFT_GIZMO_SLOPE_LIMITER_FACE_H
+
 __attribute__((always_inline)) INLINE static float
 hydro_slope_limit_face_quantity(float phi_i, float phi_j, float phi_mid0,
                                 float xij_norm, float r) {
@@ -127,3 +130,5 @@ __attribute__((always_inline)) INLINE static void hydro_slope_limit_face(
   dWj[4] = hydro_slope_limit_face_quantity(Wj[4], Wi[4], Wj[4] + dWj[4],
                                            xij_j_norm, r);
 }
+
+#endif /* SWIFT_GIZMO_SLOPE_LIMITER_FACE_H */
diff --git a/src/hydro/Gizmo/hydro_unphysical.h b/src/hydro/Gizmo/hydro_unphysical.h
index 517e3e0918ad340580e270477c0a166590546850..1a8bf334f8b6409bd757271812d4d7dc4e297a45 100644
--- a/src/hydro/Gizmo/hydro_unphysical.h
+++ b/src/hydro/Gizmo/hydro_unphysical.h
@@ -52,4 +52,4 @@
 
 #endif  // defined(GIZMO_UNPHYSICAL_ERROR) || defined(GIZMO_UNPHYSICAL_RESCUE)
 
-#endif  // SWIFT_HYDRO_UNPHYSICAL_H
+#endif  /* SWIFT_HYDRO_UNPHYSICAL_H */
diff --git a/src/hydro/Gizmo/hydro_velocities.h b/src/hydro/Gizmo/hydro_velocities.h
index eef5d408e66ea6b4bb1be0cf53452a81ea0b4e89..549d8eac3485efb9aa0b2a5ddae03b8c00fe0f8a 100644
--- a/src/hydro/Gizmo/hydro_velocities.h
+++ b/src/hydro/Gizmo/hydro_velocities.h
@@ -152,4 +152,4 @@ __attribute__((always_inline)) INLINE static void hydro_velocities_set(
   }
 }
 
-#endif  // SWIFT_HYDRO_VELOCITIES_H
+#endif  /* SWIFT_HYDRO_VELOCITIES_H */