I don't have much experience with Termux, apart from playing with it a bit. However, I have used GNURoot, a similar program, a fair bit (contrary to the name, it does not require the device be rooted). That gives you a terminal (GNURoot does work with XSDL, an X Server, but I found it too slow to be usable on my device). GNURoot uses the Debian repos, so you can get a number of Scheme systems from the repository, though at slightly outdated versions. I have installed both Gauche and Racket that way, and I'd be pretty willing to bet that Gambit might be available too.
Hope that helps -- v