It's possible to invade the UK without first taking Gib - it all comes down to whether or not the Allied player fortifies the UK. I'm in a game now in which my opponent sent units out of the UK to join the fight and left me an opening. The UK will fall soon. But of course this allows the USSR to DoW which they've done. But I think it's a good trade-off - the Axis can win every time as long as they fight a 1 front war.