I'm gonna go against everyone here.
Hands down, the best spray paint I've ever used is Rustoleum 2X cover, from home depot.
With NO primer whatsoever.
Covered that with Rustoleum automotive clear coat spray can, also available from home depot. Best clear coat I've ever found. Gas resistant and compatible with most paints.
Painted tanks with it and splashed gas on them with no problems.
I have a sandblast cabinet so I just paint right over the bare metal.
Painted 2 motorized bikes, my 50cc motorcycle, and at least 10 bikes. All of them are highly scratch resistant, brilliant, long lasting colors.
The one bike I did prime, using rustoleum primer, was the worst paint job I did. The paint never seemed to harden, scratched just by breathing on it, and just plain sucked.
I think they finally got the formulas right. I've painted tons of bikes using every spray imaginable, and the 2X cover is so much better than anything I've ever used.
In fact, I've went back and sandblasted parts that I recently painted with it, and it was so difficult to remove I had to wipe it down with paint thinner to loosen it up so the sand would strip it.
I'm totally sold on it. Before I discovered how good it was, I woulda given the same advice as everyone above, but not anymore.
The GT2 is aluminum, right?
If it is or not, doesn't matter. If it has the factory paint on it, I'd wash it, dry it completely, and paint right over it.
One can of the 2X will cover a frame, but I'd buy two anyway.
Seriously, the bikes I've rattlecanned with the stuff have come out better than factory, and as good as any professional job I've seen.