If you never thought it was possible, then you better think again. This guy filled his entire 2.7GB GMail account in less than 24 hours. He did intentionally fill his account by repeatedly sending himself a 7.81MB text file, but he originally tried to signup for all of the spam that he could.
He started by hitting forums and asking for people to spam a GMail address that he created just for this experiment. After having little to no success of receiving spam, he resorted to signing up for every offer he could find on the web. After a little bit he realized that this was not getting him anywhere because he would have to receive 675,000 emails. He turned to repeatedly sending himself a 7.81MB file. After getting within 1% of reaching his limit he noticed that the quota text, which was originally green, turned to red.
That is it, they didn’t even give him a “Woo Hoo! You filled your GMail account!” message. He was quite disappointed by the lack of an exciting message other than the one he would receive when trying to send an email to the full email account. I guess it proves that you really do get as much space as Google says that you do, but no more than what they say!