I play around with this a bit, a few things I learned is that XCode 5.0's llvm crashes when compiling Gambit-C 4.7.0's generated C code. The beta for 5.1 has a fix. Here are a few demos that might help a few people:

scheme REPL over TCP using chibi scheme: https://github.com/clarkeaa/Scheme_iOS_REPL

calling in and out of Gambit-C: https://github.com/clarkeaa/HelloGambit

You can grab a precompiled Gambit-C library from the later project if you want.