Well, the only difference you see is that you call GC_malloc() instead of malloc() (and that can be redefined, even link-time), and that on some platforms (at least Cygwin and AIX, maybe also Solaris) you need to call GC_INIT() from main().

Yes, but that should be placed in the C code, not in the Vala sources. Assuming it's a c#/Java-like language, the programmer should be spared from this job.

In case of Vala I think that required modifications are pretty well limited to underlying runtime library (gobject) and do not touch compiler too much. But on the other hand they will be quite invasive across whole Gtk/Gnome platform.