You'll do just fine with Regular (87) in your GT, but Midgrade (89) is whats recommended for it.
Premium (91/92) is recommended for the GT-S.
Since 2000, I normally use Midgrade (89) in my 90k/mi GT for normal city driving. But when I know I'll be doing distance driving -- anything where I'll be on the highway for more than a few hours, I'll fill up the GT with Premium (91/92), and continue to fill it until I know I'll be doing just regular city-driving with it again. Yes, I am aware that I have pretty low mileage for a now-20-year-old vehicle. It doesnt get driven as often as it use to, but the engine is still in excellent shape.