Totally agree. GTPs are amazing display snakes but a 20gal isn't the right size/dimensions for one. Now if you used that for a neonate and got something bigger for it as it grew, then you might be on to something. But, again, humidity would be an issue in a tank.
I also agree that a corn would be a good choice. BHB is fine and there's also Henderson Herps. They have some really nice morphs. As someone else mentioned, check the BOI threads on here, kingsnake, and fauna before you buy....
If you're interested in something other than a snake, a pair of crested geckos would be fabulous too.... ;)