how to tackle Bg4 counter against King's Indian defense ?


You shouldn't fear it if white wants to trade his dark bishop for your f6 knight.
If he trades, your dark bishop will become stronger (as it'll no longer have an opposing dark bishop to challenge it).
And if you're really worried about keeping things neat and tidy, you can put your other knight on d7 and use it to recapture if needed.

Why do you need to save the knight? The bishop is one of the strongest pieces for black's usual plans of counter-attack on the king side. Happily trade the pieces and keep attacking on the queen side - your opponent voluntarily gave up his counter-play and will have to fight on the queen side, where you have a strong advantage.