diff --git a/util/crossgcc/buildgcc b/util/crossgcc/buildgcc
index 3d570e009c..66fdf91c07 100755
--- a/util/crossgcc/buildgcc
+++ b/util/crossgcc/buildgcc
@@ -662,9 +662,7 @@ build_GMP() {
 	fi
 
 	# shellcheck disable=SC2086
-	# Bug: GMP currently does not support being build by GCC 15 (--std=gnu23)
-	#      Remove CFLAGS once it works again.
-	CC="$(hostcc host)" CXX="$(hostcxx host)" CFLAGS="-std=gnu17" \
+	CC="$(hostcc host)" CXX="$(hostcxx host)" \
 	../${GMP_DIR}/configure \
 		--disable-shared \
 		--enable-fat \
diff --git a/util/crossgcc/patches/gmp-6.3.0_fix-c23-prototypes.patch b/util/crossgcc/patches/gmp-6.3.0_fix-c23-prototypes.patch
new file mode 100644
index 0000000000..4d8105bcfb
--- /dev/null
+++ b/util/crossgcc/patches/gmp-6.3.0_fix-c23-prototypes.patch
@@ -0,0 +1,100 @@
+Resolves compilation errors with GCC 15+ in C23 mode by:
+. Adding explicit function prototypes for `g()`
+. Modernizing K&R-style function declarations
+. Fixing void parameter signatures
+--- gmp-6.3.0/configure
++++ gmp-6.3.0/configure
+@@ -6567,13 +6567,39 @@
+    1666 to segfault with e.g., -O2 -mpowerpc64.  */
+ 
+ #if defined (__GNUC__) && ! defined (__cplusplus)
+-typedef unsigned long long t1;typedef t1*t2;
+-void g(){}
+-void h(){}
+-static __inline__ t1 e(t2 rp,t2 up,int n,t1 v0)
+-{t1 c,x,r;int i;if(v0){c=1;for(i=1;i<n;i++){x=up[i];r=x+1;rp[i]=r;}}return c;}
+-void f(){static const struct{t1 n;t1 src[9];t1 want[9];}d[]={{1,{0},{1}},};t1 got[9];int i;
+-for(i=0;i<1;i++){if(e(got,got,9,d[i].n)==0)h();g(i,d[i].src,d[i].n,got,d[i].want,9);if(d[i].n)h();}}
++typedef unsigned long long t1;
++typedef t1 *t2;
++void g(void) { }
++void h(void) { }
++static __inline__ t1 e(t2 rp, t2 up, int n, t1 v0)
++{
++  t1 c, x, r;
++  int i;
++  if (v0) {
++    c = 1;
++    for (i = 1; i < n; i++) {
++      x = up[i];
++      r = x + 1;
++      rp[i] = r;
++    }
++  }
++  return c;
++}
++void f(void)
++{
++  static const struct { t1 n; t1 src[9]; t1 want[9]; } d[] = {
++    { 1, { 0 }, { 1 } },
++  };
++  t1 got[9];
++  int i;
++  for (i = 0; i < 1; i++) {
++    if (e(got, got, 9, d[i].n) == 0)
++      h();
++    g();
++    if (d[i].n)
++      h();
++  }
++}
+ #else
+ int dummy;
+ #endif
+@@ -8186,13 +8212,39 @@
+    1666 to segfault with e.g., -O2 -mpowerpc64.  */
+ 
+ #if defined (__GNUC__) && ! defined (__cplusplus)
+-typedef unsigned long long t1;typedef t1*t2;
+-void g(){}
+-void h(){}
+-static __inline__ t1 e(t2 rp,t2 up,int n,t1 v0)
+-{t1 c,x,r;int i;if(v0){c=1;for(i=1;i<n;i++){x=up[i];r=x+1;rp[i]=r;}}return c;}
+-void f(){static const struct{t1 n;t1 src[9];t1 want[9];}d[]={{1,{0},{1}},};t1 got[9];int i;
+-for(i=0;i<1;i++){if(e(got,got,9,d[i].n)==0)h();g(i,d[i].src,d[i].n,got,d[i].want,9);if(d[i].n)h();}}
++typedef unsigned long long t1;
++typedef t1 *t2;
++void g(void) { }
++void h(void) { }
++static __inline__ t1 e(t2 rp, t2 up, int n, t1 v0)
++{
++  t1 c, x, r;
++  int i;
++  if (v0) {
++    c = 1;
++    for (i = 1; i < n; i++) {
++      x = up[i];
++      r = x + 1;
++      rp[i] = r;
++    }
++  }
++  return c;
++}
++void f(void)
++{
++  static const struct { t1 n; t1 src[9]; t1 want[9]; } d[] = {
++    { 1, { 0 }, { 1 } },
++  };
++  t1 got[9];
++  int i;
++  for (i = 0; i < 1; i++) {
++    if (e(got, got, 9, d[i].n) == 0)
++      h();
++    g();
++    if (d[i].n)
++      h();
++  }
++}
+ #else
+ int dummy;
+ #endif