This isn't 100% settled, but it mostly depends on if a consumer could reasonably mistake your game for GTA, and how close it's copied.

Get The Auto 2 is very likely illegal, but because the logo is deliberately designed to be deceptive. See how theII has that strike across, to make it look like "theft"? And the acronym is the same, and the last word.

If I were a betting man, I'd bet on a successful suit on the part of the plaintiffs. It's a tough case, though.

If not deceptive, style itself is not clearly copyrighted; there are just issues with brand identity/trademark if the copy is too close that it will confuse people.

That's usually not the case, though, and it almost certainly won't be unless you do it deliberately.

See the suit between EA and Zynga:

http://www.gamespot.com/articles/ea-vs-zynga-suit-settled/1100-6404013/

This was a big question a couple years ago, and is still in some sense unsettled, but it looks like the courts really don't want to be bothered with this stuff, so you're probably safe. Few if any game companies would dare to bring up the subject again.

As far as game design, there are also patents to consider, but those are almost never enforced and have questionable standing.

I think it depends how aggressive and willing they are to protect their IP. Nintendo for instance is rabid and brutal about any character that remotely looks like zeldas link. If your character is wearing green with a shield they threaten to sue. really.

By copying I mean for example that a designer designs my characters (so different characters, not the same characters from GTA) in a background BUT having the same characteristic feel to it as in GTA San Andreas.

It is an area of civil law, and it is decided by the judge.