The Tagalog abbreviation Gg used to be short for Ginoo (gentleman, mister). But these days, G. is preferred for Ginoo because GG has come to mean galunggong (a fish) and even gago (stupid).
G. Mendoza = Ginoong Mendoza = Gg. Mendoza
GG = galunggong = gigi
a popular local fish
GG = gago
a stupid man