Gilneas plays much like Arathi Basin, but with three flags instead of five. There are some minor differences.
Zerg a Lot
In Gilneas there's almost always defense, so it's difficult to ninja a flag. And there's a lot of junk lying around that hides opponents from view, so solo players are easily jumped.
Rezzers should always group up before they move out.
If you rez near one of your flags, go there.
If you rez somewhere else, either wait for more rezzers so you can group up, or head for one of your flags.
Have an Opening Move
Drop a defensive team at your home base and send everybody else on the first zerg.
If you meet no opposition when you arrive, drop a defensive team and move your swing team halfway back. Then wait for incoming.
If you meet the other team, the outcome will give you a good feel for how the battle will play out.