I totally agree with all this. Horizontal space IS important - but vertical space cannot be ignored. 12" tall is not enough. A gtp should have at least 24" in height. And their care is different from a Ball python and there will be heat and/or humidity issues with one or the other (or both) if you try to utilize your T8 like that.
No I don't think so at all. See above. And I'd like to know what caresheets you're looking at because NONE of the ones I'm familiar with for gtps say anything remotely like that. Some may mention horizontal space, but none say 12" vertically is sufficient....
Agree with all this too. 2x2x2 is the minimum for an adult - 3x2x2 is better. Babies do fine in an arboreal tub or 12x12x18 but do need bigger as adults.