Well, you're wrong
Paper does have grain, in the sense that wood has grain. There is a slight but significant difference in the paper structure along and across the fiber direction. And when it swells, it swells more in one direction than the other. Then when it dries again, it shrinks more in one direction, causing it to curl.
Over time the differences settle out, which is why most paper eventually goes flat(tish).
Putting gelatin on this makes it even worse, as the drying rates of gelatin and paper are very different. Gelatin dries faster, causing paper curl again. The direction of the grain in the paper determines how
But gelatin has an equilibrium water content that is higher than that of paper. So at first the gelatin is drier, causing it to curl emulsion in. Then the paper dries, curling the other way. If there were any water spots on either side of the paper, you get "bulges".
More precisely the paper absorbs a lot of water without swelling much, in the intersices between the fiber. Swelling takes a bit longer. The gelatin absorbs water ONLY through swelling. So that water takes longer to absorb. and longer to release.
But the fibers in the paper will also absorb water and swell - and that, since the fibers are surrounded by water - at least in the early stages of drying - takes even longer to dry out...
So the curl is caused by the different swelling characteristics of paper and gelatin, and controlled by the direction of the grain of the paper.
Note that most photographic paper prefers to curl along the long side, into a tall cylinder. That's on purpose, paper is almost always made so that it tears more easily along the vertical than the slightly more vulnerable right across.
The only exceptions to this I know of are some small odd sizes which can go in any direction. I think Fotokemika Zagreb makes them from scrap...