Battle of Gilneas is after TP/WSG my favorite battleground, before blizzard changed the spawnpoints it was really impossible to win after you were 1 base down because people could just spawn-rush you pretty hard making it impossible to take a base off the opponents. Luckily once they changed this it very possible to win this even after overcomming a wipe in the first fight

In this battleground it is the best to have your war/feral defend GM/LH and the rest of your group will go WW. If you are horde, send 2 people with waterwalking around the back of WW incase they manage to slow your maingroup down and are trying to (smokebomb)-cap WW, these people should preferably be a dk-healer or atleast have waterwalking. When you reach WW you need to start wiping them asap, this first fight is really important. Make sure you wipe them before they wipe you, again your DK should be keeping an eye on the flag to avoid them ninjacapping it. This fight can last for quite long, but even if you notice your getting wiped, keep going, you can still recover if you all die.

Once you control WW, send your hpal to the mines/LH with the prot war and have your mage/rdruid stay at WW. The rest of your dps+resto shaman should be on the little ramp right next to the waterworks (in between mines and waterworks). So if they choose to instead of going for a base go for the inbetween group, they can very easily go trough the backway to either mines or WW. If they choose to engage the group standing there it is really important that you donot get caught, as this will certainly lead to your death. Also never try to pick fights away from bases. If they go for the WW and kill someone, he will spawn at the mines, if he has 10 second left and is considered a capable base defender (see the list I posted previously) your prot warrior can then move WW to prevent them from capping the base and thus to buy you some time, you do need to coordinate this properly though.

If they win the first fight over waterworks, you need to make sure that you donot panic, have everybody wait for their ress and buff up, you still have plenty of time. Then move towards the big mountain in between LH and Mines. See where their in-between group is and engage that one. Most likely you will kill 1-2 of them and they will all push back into WW, this is excactly where you want them. Have your DK(s) stand in between the WW and the LH to slow down any attempts from the enemy team to go LH and have your entire group push LH. There should only be 2 people there which are easily wipable so you can cap that base. If it does turn out to be a really long fight at LH you can still wipe them, considering you already killed 1-2 of them earlier. If this strategy fails just rinse & repeat till you get it right, it does take some discipline from your group to not chase them to WW here.

Always make sure your defenders are communicating with you wether or not their bases are under attack this is again of key importance. For defending against a rogue/hunter attacking LH/Mines, the same strategy(s) apply as on eots.