Dear Josep M. Arques,

Also, I would like to know if it is possible to call functions in
GAP from other languages (for instance, C).

do you want to call a GAP function from a C program or a C function
from a GAP program? There is no *direct* support for either. The
communication between stand-alone programs and GAP uses either files
or pipes. The ANU PQ is an example of a stand-alone program which
does both: GAP is used to construct a presentation, this presentation
is then writen to a file and used as input for a ANU PQ run. During
this run the ANU PQ calls GAP (in order to compute orbits) using
pipes. The output of the ANU PQ is writen to file and read back into
GAP.

best wishes
Frank