That is not correct. Only LAND units may take control of the now uncontrolled Neutral. The allied plan may land there but it does not take control.
Global 1940 Europe Second Edition Rules:
Page 11 (BOLD added by me)
If the attack upon the formerly neutral territory is unsuccessful (the territory is not captured), any remaining defending units stay in the territory but can’t move. The territory remains uncontrolled (place a national control marker on it face down to indicate its new status), but units from the side it’s now allied with can move into it and take control of it and its remaining units in the same way as if it were a friendly neutral.
Page 10 (BOLD added by me)
Friendly neutrals may not be attacked, and air units may not fly over them. They can be moved into (but not through) as a noncombat move by land units of a power that is at war (see “Noncombat Move,” page 22). This moves the territory out of its neutral status, however. The first friendly power to do so places its national control marker on the former friendly neutral territory, and its national production level is adjusted upward by the value of the territory. With the territory’s loss of neutrality in this way, its standing army is immediately activated. The units placed belong to the power that now controls the formerly neutral territory, and may be used freely beginning on that power’s next turn.
Also, the land unit cannot be an AA Gun though I am unable to find that right now. It might be in an FAQ.