Instead of using trim in the transmitter, you should adjust the length of the linkage. The normal procedure for setting up the tail is to put the rudder gyro in normal (rate) mode and then put the helicopter into a hover. If the tail drifts, mechanically adjust the linkage until you have no left or right drift. After you get the tail set up mechanically, you can then switch the tail gyro to heading hold mode.
If I understood your original post, it sounds like what you are seeing it that the tail is drifting to the right as you try to hover? It almost sounds to me like you have the gyro in rate mode because (from what I've seen anyway) the tail has to be set up way off for it to drift in heading hold. This might also account for why you're not seeing much difference when you change gain values?
I'm just kind of stabbing in the dark, here, based on the little information you've posted so far. It would probably be easier for the guys to help if you could post some more information about your VBAR settings, what radio you're using, etc.