So GSCs are really good at 3 things
1) Massive attacks, insane amounts of them; but they are low str and short distance, str 3 and str 4, 8-24" range
2) Insane DSing abilities
3) Massive amounts of bodies
So you will need to do 3 things
1) Have fly and out move and out deploy your opponent (like others have said)
2) have massive anti infantry with some anti-tank (Venoms + blasters and Ravagers with Scourges and Shredders)
3) Finally speed, make sure to stay away from their 2-3 good melee units and their really good shooting units, like the Bikers and Shotgun guys.
I would kill the Bikers first if they take any.
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Dark Eldar 10k+
AoS: BoC 8k, CoS 3k