How big is the photo?
Some of the surface finishes on Gatorboard aren't dry mount friendly. The white is like a very thin melamine and is hard to get things to bond to it. The craft paper colored finish is easier. Gator is loaded with bad chemistry for art, so I tend to shy away from it. I'll use it as a support board if I can wrap it in metalized plastic film to contain the off-gassing.
How about ACM panel? Kool Tack makes precoated ACM called Competition Plate. I've used it to mount inkjet output of large photographic images. The temperature needed is pretty low, but I erred on the side of caution and ran it at the max temperature suggested, then turned off the heat and let it cool under vacuum (about 20 minutes to get it down to 90F). It can be trimmed with a utility knife and a straight edge, but you will want to burnish the one side after cutting and smooth it with a file so the edge isn't razor sharp.
ACM doesn't work so well with Fusion or Colormount because of the temperatures necessary to get a good bond. The core material will soften at those temps.