It could be good to have a small client along with it, just like lib bearssl have brssl. At least for testing if works across the whole chain.

If libgbt needs other libraries, this opens a library chain: someone writing a wrapper library over libgbt to build its client more "dev friendly", which is in turn used in a GUI widget to stream videos used in a larger program.

What about an open and free implementation of sha1.c and md5.c in the repo ?

Maybe it could not support md5 at all as it is quite deprecated in the RFC.