a good pacer won't rely on GPS. they are a runner that can dial into a pace and stick to it. They also understand to back off when going up hills and speed up on downhills.
GPS isn't always right, and you end up screwing a lot of people relying on you if you lose your signal, your battery dies, or you accidentally hit something that resets your watch.
you need to be comfortable pacing. Its a whole different type of running. There are some fantastic pacers out there and some really terrible ones. You should know your strategy ahead of time and communicate it to the group. I've seen pacers negative split, positive split, run even splits, and ones that are all over the place.
not bagging on anyone, but races a lot times will take what they can get in the way of pacing volunteers. Just because someone is faster than the pace they they are intending to hit doesn't mean they are a good pacer. If someone is interested, i'd suggest they practice running that pace several times before race day. Sometimes it is hard to run "slow."