In an aquarium the water is pumped straight up to the tank, this is the head height.
In a closed loop system, water returning down exherts a force due to gravity equal to the force required to lift the water (think of syphoning, as long as the outlet is equal to the inlet theres is equal force of gravity, lower or raise the inlet or outlet and you will have flow regardless of the rest of the tubing being higher or lower, as long as it stays in the water). Gravity doesn't exhert an effect on a closed loop, the pump just circulates.
You are correct in that GPH is not velocity, but it is the effect that the velocity of the water in the tubing has against restrictions which causes friction. With one elbow there probably is the equivalance of restriction that a 1' head would create, which in turn lowers the velocity and GPH. The more elbows you add the less the effect becomes as the first one already has lowered the flow rate enough to make these restrictions easier for the water to pass threw. Meaning if one fitting dropped your flow by half, a second would only drop a little more. By the time you get to five or so restrictions of the same size, there is hardly a noticable decrease in flow.
Increasing tubing and fitting size makes all these restrictions lesser. In fact the move from 3/8" to 1/2" more than doubles it's ability to flow from primarily the fact that there is less friction threw the fittings and the tubing, and the water doesn't need to travel at a high velocity to achieve this flow (again restrictions + velocity = friction / restricted flow). But once again there is a large benefit like you said from going to larger tubing.
P.S. I edited my post as I don't believe the last comments where appropriate, sorry, we're all here to learn.
