I would recommend turning it off, unplugging it (to be safe) and ensure
that all chips (especially the video circuitry) is firmly seated on the
board.
It is my guess that either something crawled under your motherboard and
died, causing a short (hey, I found a dead roach in a floppy drive once
upon a time.), something is not making good contact, or the video
eeprom is just bad and needs to be replaced (assuming the //gs uses one
like the other // series machines do, pardon my ignorance if it doesn't
-- I don't have a //gs lying around to look at for reference)
-B