I have done more work to build/run GIFT on Debian Bullseye.
There is a Docker image where you can see it working. It is paired with monosock's PHP MRML client.
See the patches here:
Review these patches instead of the ones on the previous e-mail.
Or look at the repository if you'd like to try it out in Docker:
The status is that it runs now, although compiler optimizations have to be disabled.
I guess recent versions of gcc are more aggressive optimizing than older versions and that makes undefined behavior more of a problem.