This is caused by JDK-8377509.

The GCC license file was added in a way which, as far as I can see, leads to it being included in every build of OpenJDK, yet it seems to be related to changes to the devkit in JDK-8351603 (possibly due to ASAN bundling for JDK-8334455?). The reason for this inclusion was very light on details.

If I build with clang on GNU/Linux:

configure:108447: Using clang C compiler version 20.1.8 [clang version 20.1.8 ( 20.1.8-2.module+el8.10.0+23372+3f2ea6fa) Target: x86
_64-redhat-linux-gnu]
configure:108573: checking for C compiler version
configure:108582: /usr/bin/clang --version >&5
clang version 20.1.8 ( 20.1.8-2.module+el8.10.0+23372+3f2ea6fa)
...
configure:111815: Using clang C++ compiler version 20.1.8 [clang version 20.1.8 ( 20.1.8-2.module+el8.10.0+23372+3f2ea6fa) Target: x86_64-redhat-linux-gnu]
configure:111936: checking for C++ compiler version
configure:111945: /usr/bin/clang++ --version >&5
clang version 20.1.8 ( 20.1.8-2.module+el8.10.0+23372+3f2ea6fa)

I still get this license included:

$ head -n1 ~/builder/26u/images/jdk/legal/java.base/gcc.md
## GCC - libgcc and libstdc++ 14.2.0

It seems like this should be restricted in the Makefiles to systems building with gcc and bundling whatever leads to the requirement for this license.