if i buy say 1 gb from bittrex today on a new account, will i get 0.2 gb added after the airdrop?
You don't get any airdrop if you leave your bytes on Bittrex.
assume btc/gbb price remains same
This assumption is wrong, right after every airdrop the price has dropped due to people selling their free coins.
i understand, but assume price remains same and i have the gbb in my wallet. the gbb balance increases by 20% automatically right?