Some people are using this paper (FAEW/SP) without any size. I think it somehow depends on your pigment choice too; my understanding is that carefully selected pigments leave no need of extra sizing with this already very hard sized paper. Never used it myself but I've heard many "yellowing" stories about glyoxal; again, my understanding was that the yellowing was caused by using too much or strong hardener and not washing / rinsing papers after hardening is over. Nevertheless, all this is to much effort for my liking, especially so in the context of gum printing - a quite straining process, already... BTW, the glue I use doesn't change the surface when diluted 1+3. (It's not Gamblin PVA size.)

Good luck,