Welcome. Be sure to introduce yourself in the "welcome lounge" so we can give you a proper hello.
Welcome Lounge

A 20 gal isn't too big in my opinion. It should provide a nice big living space for them to explore.
It sounds like you probably know but a 1 to 10(bleach to water) part bleach solution will do a very good job sanitizing their new home.
Here is a link to the forum care sheet which can be very helpful.
Garter Snake Forum - Garter Caresheet