First, "gawapi" is a strange name, and I could not find it if grep'ing all
files in the source hierarchy. Files with the more sensible name "gawkapi"
are there, though; you'll find gawkapi.c, gawkapi.h in the source directory
of the standard 4.1.3 source distribution. Now I'm not sure whether you are
referring to that version or something else. In former times there was a
special variant derived from gawk called xgawk (with XML support). Now you
find XML support available in the main branch of gawk through an extension.
Janis