Try spraying the back of the print with a solvent based workable fix before using a wet process. This will prevent the paper from absorbing moisture and buckling.
Gesso dries too hard. You need to use a polymer acrylic gel which remains sticky when dry. Coat both the panel and the print. At this point heat will reactivate the polymer which is essentially a thermal plastic, Or merely add another coating of polymer and stick the sandwich together while still wet and weight it down for a day or two.
Adamphotoman aka Grant
New Brunswick Canada
BetterlightSuper6KHS, medium and large format lenses, Sinar hybrid 4x5 cameras[17lenses], D800,16fish,14-24,17-35,28shift,45,60micro,85tilt,105micro,135DF,200mic ro,ipf8300 printer Watercolour,softpastel,acrylic,airbrush,oil stick expert art reprography,finishing Giclée,stretching canvas,collects kolinsky watercolour brushes