I can't recall the reduction in our GEM gear boxes for our arm, but we have a shamefully heavy intake system that is at the end of an arm that pivots at the back of the robot. It can lift the intake fairly well, as well as the portcullis.
This is the practice session we had yesterday and you can see that we have a similar design using two sets of GEMs and CiMs. We didn't do much lifting during practice since we didn't have our encoders and limit switches programmed yet, but it does lift. PM me and I can get back to you on gearing specifics.
It would be worth giving this a shot. Do you have any sort of lifting assist? Our arm has two 40 pound gas springs keeping it up, along with the plate springs on the intake pivot. I highly recommend a lifting assist of some sort. The shock absorbing effect of the gas springs and the plate springs also help the arm come down gently as you can see when we get some airtime on the moat. You might want to consider the stresses that your arm will endure when doing some bumpy driving. I can almost guarantee it will pop up and slam if you go too fast over a defense. More than just your hearts will be broken.