Gravity doesn't matter. Since the outlet (front) and return (rear) of the transmission are at the same height, there is no pressure change from the up and down. That is why a siphon hose works.
It doesn't matter which way you flow thru the cooler, at least for the ones I have seen. Actually, there is a slight theoretical advantage in going it at the top and returning at the bottom, if your cooler has horizontal tubes. That is because the air is slightly warmer at the top. It is a heat transfer effect and has nothing to do with fluid pressure (or height). Lookup "cross-flow heat exchanger", but surely doesn't matter here.
A bigger question is whether you want to keep your radiator tranny cooler in series. That helps a little, but has the disadvantage of risking oil in the coolant or (worse) water in the tranny. I decided not to. If you do, probably best to run thru the radiator cooler first. As I read, there is no risk of over-cooling the tranny fluid unless you live above the Arctic Circle.
