I don't think you're missing anything about gas springs.
The last time we had an arm (2011), our goal was to be able to score on both sides of our robot (our arm was designed to rotate more than 270 degrees).
We had two gas springs that we bought (maybe ordered) from our local hardware store. We had attempted to connect the arms to the springs in such a way that the springs would perfectly cancel out the force of gravity. We then used a (not very powerful) window motor, which in theory should have been able to effortlessly lift our arms.
Theory only works when the math is done correctly. If anyone looks at our match statistics that year, they will find low scores.
Because our arms were not properly balanced, our window motor would overheat, and we'd spend some matches without a working arm, and some with our our arm straight up!
Used correctly, gas springs can offset the force of gravity on many mechanisms (like arms)
