GT1 and GT2 are like the Base and GT of my year, I've got a 2005 GT. It just adds a few "luxuries" like power seat adjustment, steering wheel radio controls, etc.
I personally use Quaker State Full Synthetic 5W-30. It's barely over $20 for a full 5 quarts at the Walmart that's close to me. I was skeptical at first since, you know, it's sold at Walmart... but about 2,500 miles in everything's great. I got about a 2-3 extra mpg on average, and it's running very smooth at about 88,000 miles.
For a filter I got a Fram Ultra. Be careful with Fram filters, as a good portion of them are junk, but after seeing a Fram Ultra torn down on another forum, I was very impressed by the build quality, and they're still very cheap.
As for oil change interval... I'm honestly not sure. The filter actually states that it is good for 15,000 miles, but I just can't feel safe doing that. I'm sticking with synthetics now, so either every 7,500 or 10,000 miles, probably do 7,500 to be safe.
Next change I'm getting two filters, putting in a new one along with some Seafoam in the crankcase to clean it out and actually catch the junk, hopefully quieting a sticky lifter in the process. You can hear the faint lifter tick after it's warmed up, and it's not constant, so I'm hoping it isn't actually a bad lifter or a loose rocker bolt etc.