Have to get air to the gpu. Without a decent supply of air, there's nothing for the fans to blow over the gpu heatsink. If nothing is moving over the heatsink, nothing gets cooled off.
So the cooler the supply of air you can get pushed to the gpu, the lower your gpu temps have a shot at being. If you can get enough air over there, you'll reach a saturation point, and thats where the actual voltages and workloads have set the amount of heat and nothing airflow-wise will change that. It is the same way with the cpu. That's when voltage changes will be of any affect.
Load temps are somewhat different. The gpu will then put out far more heat with the upscale in power use, and while airflow has an affect, it's nowhere near the affect that power plays on the temps.
1w can change 1cubic foot of air about 3°F in 6 minutes. A normal mid size atx case is around 1cubic foot in volume. So with no airflow, a gpu at idle could raise that air @ 3°F in 1 minute.
A 50cfm fan (120mm) can move 50 cubic feet of air in 1 minute. Or about 1 cubic foot of air per second. So figure a pair (intake/exhaust) will totally refresh a case with outside air completely in less than 2 seconds. A 10w gpu at idle raising 3°F in a minute doesn't stand an ants chance of heating up a case. Even if the fans were spinning at 50%, that's still 5-6 seconds for total refresh, all heat gone.
Without good airflow, a case becomes an oven. Because it's physically impossible to cool an object to below ambient temps by mechanical means (fan blowing air is mechanical) if your case is reaching 50ish °C inside, the gpu and cpu has no hope of being less, and because of workloads is usually @ 10ish°C more.
With good airflow, a gpu/cpu should be somewhere around 30-35 if the room is 20-25°C ambient.