The source consists of four files: FLink.c, gateToFermat.h, gateToFermat.c and gateToMath.tm.
The gateToFermat is used to access the Fermat program.
The gateToMath is written in a meta-language for Mathlink and is compiled with mprep.
FLink.c uses the mathlink.h header file that comes with Mathematica.
For the reason that gateToFermat uses fork(), FLink is not available under Windows.
Download the sorce here.