Some were great but most were terrifying - and so spooky too.
The title says it all : Gaby is not an acronym despite that it first stood for "GTK Address Book of Yesterday" (and after for "Gaby Astute Base of Yesterday"). In fact I liked the first one but had to change when Gaby evolved in something else than just an addressbook and I never really loved the new one.
That said, what does Gaby stand for ? Well ... nothing. Gaby stands for nothing. Why would you want an answer to every questions ? Gaby doesn't stand for anything including references to friends (Gaby is a common nickname for Gabriel and Gabrielle in French), to an old cartoon featuring a teddy bear named Gaby, to the song 'Gaby oh Gaby' from Alain Bashung (French singer), to ... 
I gave the possible meanings people sent me, there are probably others