To accept such a white flag in RL, one must assume the pilot "waving it" (gear down), is in a situation where he can land in you'r territory and surrender,
as in escorted to a friendly airbase or ditching in a friendly to you, territory. For eksampel a german pilot badly damaged over england surrendering to another english aircraft, during ww2.
If for eksampel the same german pilot was to drop his gear deep in german territory, and expect to be spared. 1st he would probably make it to some, to him friendly airfield, saving his life and the plane or at least spares. So he is actually not able to surrender.
Logic dictates, over your own country and no threats near, accept the surrender.
any other situation, shoot him down. thats whats you'r up there to do.
Now in Aces High, you can not actually surrender because you can not land on a enemy airfield unless all ack is down. the only thing you can do as in a surrenderable situation, is to land in unfriendly terretory and getting captured.
So if i saw a enemy plane dropping gear, near battleline,or in his own territory, i would shoot him down.
If deep in my own country, i would let him ditch, mostly not to killsteal.