Full install 125Mb for 5.99 USD... So, did Gog.com just sold me the shareware version of the game?? Where do I go to purchase the full CD version?

Edit: This is indeed the full version. After completing Episode 1 that has Stages 1 to 3, I saw the original 3D Realms ad for the full version, and I thought: wow, this is only part of the full game. But then, when I tried to start a new game, I saw there are 3 Episodes. So, to play Episodes 2 or 3, I have to go to Start New Game, and choose Episode 2 to play Stages 4 to 6 progressively, or choose Episode 3 to play Stages 7 to 9.

Terminal Velocity is a nice game, as I get to flip the ship upside down, to enter tunnels, or use an After burner. It does feel a little like Descent 2.

I agree.
I own the "Limited Edition," non-shareware cd which claims the full game "Includes 3 unique planets with 3 huge levels each." From your description and GOG's the GOG version seems to be the complete version.
Edit: sorry, didn't notice the date of that post.