As Renegade Paladin said, GK Force Weapons are just like normal Force Weapons in the main rule book. The only exception is that the entire unit, barring ICs, makes only a single, combined roll to apply them to the entire unit's attacks.
Not sure how he managed to kill 11 scarabs though. Even 5 GK Terminators with Falchions and a banner are only throwing out 15 Force weapon attacks (plus 3 normal attacks from the guy with the banner).
There are a few limitations, so make sure your opponent wasn't forgetting them. If he was, and he was doing it on purpose, start throwing things at him.
- Activating Force Weapons counts as using a psychic power, so he can't use Hammerhand in the same turn. This means that their attacks will be S4 (or S8 for Hammers, which ID Scarabs anyway).
- Still has to roll to hit and wound normally.
- Unless there's a Brotherhood Banner in the unit, he has to roll to activate the weapons on Ld 9. Even then, the guy holding the banner is only going to be throwing out 3 non-Power Weapon attacks at I4 WS4 S4.
EDIT Ninja'd and:
Not quite. There aren't just 3 Psykers a unit. Every GK is a psyker. All of them. Even the guy stuck driving the Rhino is a Psyker. The Dreadnoughts too. The Brotherhood of Psykers rule that they have means that units of them act like a single psyker though, making psychic tests together, only allowed to use a single power and so on. So 11 GKs, which I'd assume would be a unit of 10 GKs and an IC, would make two tests, one for the unit, one for the IC.
So yes, every GK with a force weapon is throwing out instant death (assuming they pass the test to activate them of course). Which tends to make assaulting them with multi-wound models and really bad idea.
There are two good ways to kill GKs in assault: Lots of single wound infantry who don't about force/power weapons OR models with good Invulnerable saves (because then GKs are really just S5 codex marines).
Whatever you do, don't send multi-wound models at them - GKs tend to be something of a hard counter to those. Except possibly Wraiths (if Wraiths are multi-wound models, not sure), thanks to their Invulnerable saves. I've not gone up against them myself yet, but GKs tend to rely pretty heavily on ignoring armour saves to win assault. A unit that gets a good save anyway might ruin their day.
You should probably note that not every GK has a Force Weapon either. PA guys with heavy weapons don't have one, nor do TA ones holding the banner. Techmarines have to pay points to upgrade from power to force as well. Attacks from those guys do not cause ID (unless attacking something that's T2, which is actually surprisingly likely with Rad Grenades).