I may be missing a rule or something, but to me Gibraltar is the lynchpin to the US invasion.
Once taken, it allows access to all of Italy, Southern France, Normandy and Holland. If the US builds up to 4 loaded transports and escorts before T4, then buys 4 each (transport, tank, infantry) at 64 IPC for a few turns, it’s very hard to defend since there’s wave of 8 pieces each turn.
If the UK ties up Italy, Germany is exposed on three fronts while fighting Russia.
I just finished a game with my son. He was pressing Moscow and had a sizable force in Normandy, but after landing in Southern France and surviving the first wave, with RAF softening from UK, the next wave hits Normandy, then the fighters go in to defend and no matter how much Germany builds on France, it can’t stop the buildup as UK bridges.
I had the UK take Denmark as a pause to keep the German Navy enclosed, but ultimately a destroyer screen by the UK outside of Denmark, plus a simple destroyer screen and roving Battleship for the US
to protect the transport cycling is enough to sustain.
Am I missing a rule regarding Gibraltar? If so, a naval base in Morocco would do the same (provided the Med was clear of Italian Navy).