Depending on which compiler you use, there should be a way to see the code after the preprocessor (which does the macro expansion, macros are not known by the compiler at all) is done.
With gcc, the option is -E. Here’s a simplified example, using toy code and not the actual GTK+ macro:
~/tmp> cat cpptest.c
#define SET_FLAGS(w, f) ((w)->flags |= (f))
int main(void)
{
SET_FLAGS(0, 4711);
return 0;
}
~/tmp> gcc -E cpptest.c
# 1 "cpptest.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "cpptest.c"
int main(void)
{
((0)->flags |= (4711));
return 0;
}