My team tried to use GTS last year on our aiming devices and it was a tremendous failure. After LABview testing the kit GTS, we purchased better (more expensive) GTS. Still it was just too difficult to mount the GTS with the tremendous precision they require and keep them steady (right next to our high vibration shooter).
I think GTS just are not rugged enough to be reliable on a FIRST Robot with all the vibration and impacts. We use USDigital optical encoders for drive and other things. I recommend the
S16 series with the Ball Bearing option. We usually just pin a plastic gear to them and connect it to a spur gear in our drive train. The B option greatly increases the max rpm and life of the encoder so you can use it for multiple years.