Dunno if it will help in your particular case, but check that you're compiling with debugging information on the C level, i.e. passing -g, possibly -O0, maybe -ggdb or -gdwarf-2 to gcc. (There are various ways to do this, from using the cc-options Gambit compiler option (I think -O0 won't help that way) to modifying makefiles [to editing gsc-cc-o.bat in earlier releases] to putting a gcc wrapper earlier in the PATH..)
Christian.