From the manual:
Gyro Gain – Turn clockwise to increase gain. Typically too little gyro gain causes the tail to slowly move around and drift. With the gyro gain too high, you will notice a very fast oscillation in the tail known as "Wag." This adjustment is not very critical so please don’t spend a lot of time trying to get "the best" setting possible.
Tail Rotor Trim – If the helicopter wants to drift one direction in a stable hover then try using this adjustment. The adjustment is very sensitive so only make small changes. Turn clockwise to add right tail rotor trim.
Comments:
Gyro Gain - I find that if I turn this too much counter clockwise (if I remember correctly) then the head is really stiff and yaw control isn't very smooth and snaps from from one point to another. I like it smoother than this so at the moment mine is at the 11 o'clock position. I haven't experienced the tail wag it's mentioning.
Tail Rotor Trim - Some say it should read "If the tail wants to drift..." rather than "If the helicopter wants to drift..." and I tend to agree. If I turn this one too much to counter clockwise (I think), then it'll sound like the tail is surging, like it's giving extra juice every other second or so but the head still keeps in position while it's doing this. This is at the 12 o'clock position for me.
It's almost as if both are working in concert and changing one affects the other - that's the impression I'm getting anyway. Make gradual changes to them (with the included plastic screw driver) to see which works best for you. You may want to mark their current positions and then start fiddling with them.