Masonite is much, much heavier than gator. Although also much cheaper.
For the big frame sizes you need a serious hanging system with heavy Masonite, whereas Gator frames usually can hang safely with 2 nails in drywall, not even into the studs. Masonite needs to be cut with a saw, Gator with a utility knife. Cutting Masonite creates sawdust. And if you are doing art fairs, heavy frames are a serious PITA. And Masonite is a pretty heavy payload for picture frame moulding, some extra bracing might be needed.
I know painters who build up sides behind Masonite with the milled 1x2 and 1x3 Oak, Pine, Poplar, etc at Home Depot and Lowes...the so-called "furniture grade" wood. It's kinda popular right now with starving artists. That leaves you with a glue joint between two dissimilar, rigid materials which might not be very stable in the long run. I saw a Masonite box frame that fell of the wall, the glued bottom piece of pine separated by pulling off a thin layer from surface of the Masonite. I suppose you could drive countersunk screws through the Masonite and into the "sides" but that might show through onto the print unless it was perfect. It's something I'd personally avoid, doesn't sound stable somehow. But I do have old rag paper, silver prints that were glued to Masonite in the 70's and they are still OK.
From my perspective, for the effort required to simulate a gallery wrap with 1x2 sides behind a mounted print, I rather just build a frame which would usually be easier. The frame will cost a little more, but IMHO it adds a lot of perceived value to the product. Some of the very narrow, deep "shadow box" frames can give you a look that it is vaguely in the gallery wrap aesthetic ballpark.