Imagine that you will have your personal keys. And after fork you will import these keys into BTG "devs" coded wallet, never known and used before. This wallet will send these keys to BTG guys... and your bitcoins will say good-bye... They will have full access to your wallet.
Premine shows the objective of this event...
that is why, you should first move your BTC to another wallet and then import the seed in an "empty" wallet to collect BTG.