I just coat the Gatorboard with MM, roll the oversized print (mostly canvas for me) over the board (print rolled around mailing tube works great for this), lay a piece of craft paper over the print, then just wipe lightly with my hands. I am impressed with how well this works. I don't get any bubbles or irregularities of any kind.
I also just started a new technique where I leave an extra inch of canvas on all the sides. After the mounted canvas dries, I brush a strip of MM about a half inch wide around the back of the Gatorboard. Then I wrap the canvas around the back, holding it in place with a piece of low-tack painters tape (home depot blue stuff). I lay a piece of glass on top of the whole thing, put a few books on top (keeps the flaps flat to the board and edges nice). Corners were the only tricky part of course but I just cut so no folding or overlap. Finished edge makes me happier to hang frameless this way vs. seeing the Gatorboard foam edge.