Gilneas plays much like Arathi Basin, but with three flags instead of five. There are some minor differences.
Hold Two, Forget Three
The game ends fairly quickly if you take two flags and hold them ... having three doesn't speed things up enough to matter. It's a lot easier to play defense in Gilneas than it is to be on offense. So, if you have two flags, sit tight and wait for the fight to come to you.
In some ways this is more like an arena venue than a battleground. You want to fight in tight groups with mutual support.
Have two defensive groups and a swing group. The swing group should ride back and forth to take care of incoming opponents.
One healer per flag is usually enough. Since the defensive groups are about the same size as the offensive groups, it's hard for opponents to focus fire on the healer without taking casualties. Also, the GY is close, so healers come right back.