Installation checks with a snapshot of GCC pre-16.
This defaults the C compiler to -std=gnu23.
The current description of changes is

https://gcc.gnu.org/gcc-16/changes.html

See also

https://gcc.gnu.org/gcc-16/porting_to.html

Althoughh those do not say so, it defaults to C++20 (like R-devel).

Other details as https://www.stats.ox.ac.uk/pub/bdr/Rconfig/r-devel-linux-x86_64-fedora-gcc

Debian unstable and Ubuntu usually have a package 'gcc-snapshot'.