These questions pop up periodically, so I have this page dedicated to a seemingly trivial item. The short answer is, all MGA body sheet metal is 18 gauge, 0.048-in thick, 2-lb per square foot, and all MGA frame sheet metal is 14 gauge, 0.075-in thick, 3-lb 2-oz per square foot (including the round tubing). You need to know this if you are procuring sheet metal for repairs. It is also nice to know when you are buying replacement sheet metal panels.

When buying body repair panels it is always appropriate to ask, what is the metal thickness? And when you receive the parts it is always appropriate to measure the thickness (and return the parts if you are not satisfied). Many current replacement panels are made from thinner metal than original parts, and some worse than others. Where original body metal was 18 gauge (0.048), many of the replacement panels are 20 gauge (0.036). If you ever find a replacement panel that is 22 gauge (0.028), you might as well send it back immediately.

Bending strength of sheet metal varies as the cube (third power) of the thickness. So, ....
(0.036/0.048)^3 = 0.422
(0.028/0.048)^3 = 0.198
Does that give you a hint? 20-gauge metal reduces weight by 25% and reduces bending strength by 58%. 22-gauge metal reduces weight by 42% and reduces bending strength by 80%. Even with the box section structure that would be at least 50% to 60% weaker in beam strength. I wouldn't put such thin sheet metal in my MGA body sills even if it was free.