The Gate is a membrane transport protein (or proteins?) that can do all sorts of things.
At the end of the series, it acted as an antiporter. Al was stuck inside the membrane protein itself throughout the whole anime (that's where his body was, in the membrane between worlds). Truth-kun is a Gate-mediating protein. Ed had to go to our world in order for Al to come back to his.
At the end of the movie, the Gate is a bi-directional symporter because Ed passes through the Gate with all the German soldiers, then comes back with Al.
Throughout the anime, I suppose that the Gate might also act as a uniporter when souls from our world are traveling to Ed's world to fuel alchemy. That might be a sort of antiport, except that I think of it as more like how cells generate ATP by pumping hydrogen ions across the mitochondrial membrane. If you think about it, it is making a gradient of souls, raising the concentration of souls in Ed's world for the purpose of fueling chemical reactions.
Science majors (and science majors-to-be) of FMA fandom, are you as dorky as me? If so, I hope this pleases you. High schoolers who haven't learned this yet and non-science majors, I hope this amuses you, too.