If the opponent is attacking you cant GB them (Ive manged to do it a few times but it was in the very first ms of their attack)
Use GB when they are not attacking or after a block/perry (free GB)
But be careful with the GB after block because they might have started a chain and you will GB them while they are attacking (which will not work)
If you are a LB you can block -> shove -> GB (they won't be able to start a new attack so fast)