The only GTs that I could find currently for sale are the two that have already been posted.
I spent over eight months looking nationwide for a 2003-2006 Vibe GT to replace the one totaled by a guy who ran a redlight.
I think that 2003 had the highest number of 1st gen. Vibe GTs produced with ~6916. There used to be an online chart that showed GT and total Vibe production numbers but I can no longer find it.
From 2003, production numbers for GTs declined to the point that Pontiac discontinued them after 2006 for lack of sales. IIRC, 1st gen. GT production was less than 20,000 total so they were never a common car.
Now that they are at, or are approaching, 20 years old, good condition low mile cars are very rare and will fetch a premium.
The excellent condition, one owner 2003 GT with 77.5k mi. I bought just before covid hit was $5000. The 2003 GT that replaced it this year was $6300 and well worn with many cosmetic, and a few mechanical, issues at 94k mi.
Prices are starting to come back down a bit, but at this point there just aren't that many 1st gen. GTs left in any condition.
Just be patient and keep looking. 1st gen. GTs do occasionally pop up for sale but at a much slower pace than in the past when more of them were still on the road.
2005 Vibe - 240k
2003 Vibe GT - 117k (totaled)
2003 Vibe GT - 111k