The compiler introduced with XCode 5 (this is the version one must use when running OSX 10.9, Mavericks) has problems to compile GAP as-is. The following steps seem to resolve this isssue for GAP 4.6. (I have not tried older versions.)
#define HAVE_STRLCAT 1to this file.
touch src/*.cand then
maketo force recompilation of GAP. You will get a number of warnings (for example about something called PIE), but the binary should work. (Let me know if it does not.)