I have a steel box tube gantry. It absorbs quite a bit of heat when cutting 1" material in a chain cut program. (lots of small parts in a tight area of plate) It never gets hot enough to make Aluminum become unstable. It doesn't get to hot to touch but does get to hot to hold on to. I don't think it is so much direct heat from the torch flame as it's heat radiating off the 1" steel that's being cut.
I agree with WSS Aluminum should be fine for a gantry. If you are using small motors & drivers Al will almost be a necessity.
If it works.....Don't fix it!