Autogal SQL is a popular choice for tgps, I happen to use it on two of mine, but before you jump to buying it you should know that there are a few others that have options that might be more appealing to you, especially if you want to start up multiple tgps.
An example of a good tgp script that can handle multiple sites would be Vanilla TGP. I haven't used this, but I have used the demo and its a tight script. For a no skim version you're talking about $400 though (they also have a free version, but it'll skim 2% of your traffic) So, depending on your budget that might be something you don't want to do. Personally I'm thinking of changing over to vanilla tgp the second I have the time for it.
You can run multiple tgps off of one install of Autogal SQL. It doesn't break the license as you're only using one database. Do you plan on using multiple domains? If you do, you can call the page with a php include and it should work out nicely (I tried that on a series of pages just to see if it would work and it did). The submission form would be a little tricky, but you can figure it out
Trade scripts, there's plenty of good free trade scripts out there and they'll take a small skim of your traffic. Some scripts I've used. Arrow Trade Script...it takes a very nominal amount of your traffic and it's a pretty robust script. TTT is another tradescript that works well, easy to install, and they take 1% of your traffic. Personally I went to a no skim tgp model, but still use the tradescript for the toplists, etc.
Designer, I have a few that I'd recommend, but not sure how busy they are and if they do TGP designs.