I can't imagine gta V ever fit on 12 gb of space (they had a model with 12gb?), but if it did that would have had to have been before updates, in which case you wouldn't be able to play online. Hard drives can be found for very cheap now. If you were good with small hard drive space all along just go grab a 250gb drive from bestbuy and swap out your ps3 drive. I know USB storage was available at one point but I could have sworn was updated in later. I never had major storage issues so I never needed to look up info on it.
@Fatboi1 knows more about this I think, as far as if you can plug in a thumb drive and use it as expanded memory for game installations. I still say just go to bestbuy and grab a new hard drive.