You mean you’re not Larry’s brother, Arthur? hehe
I was looking in the rulebook and I didn’t see where it said eligible territory, Leatherneckinlv. What it says is that you can place an airbase in any “controlled” territory. When you liberated Gib you returned it to being a UK controlled territory and not a US territory so that’s why you couldn’t build an airbase there. If London was under control of Germany or Italy then you would’ve been able to take control of Gib.
My guess is this is what prevents players from, for example, moving an American infantry unit to a Russian territory and then placing a complex there. If that were the case you could then move the same infantry unit to all of the 2 IPC Russian territories that don’t already have a complex there and then place a US one. I know that there is a difference between airbases and complexes but in the rules you have to control a territory to place anything there.