To be fair, GTA5 is not a bad game, so I wouldn't say that people who bought it have poor taste.
That being said though, since FF7 is a PS1 game, and the vast majority of those sales were racked up during that era, it's in many ways just as impressive as GTA5 in its own right though, even if the sales aren't that great by contemporary standards.
1. The gaming public is much larger now than back then, meaning that multi-million number sales means a whole lot more in that context.
2. Gaming prices haven't changed all that much since the PS1 era (at least where I am from), whilst game production costs and inflation has sky-rocketed, meaning that percentage-wise, FF7might be the more successful game from an income perspective.
FF7 came out at a time when game development was less costly (off course, devs had less money to spend as well), when fans weren't as numerous, and in a new market where they (Squaresoft) didn't have the same reputation or reach they have today (much less what Rockstar has today).
It truly is an impressive feat.
The success of GTA5, being the next big thing from Rockstar after a long line of other successful products in the same franchise, with its reach through the internet and social media, and the increase of participation in gaming and gaming culture etc. isn't all that surprising or impressive really - the same way the movie-version of The Hobbit's success after Lord of the Rings, isn't all that surprising or impressive.
