Gen 1

Generation 1 stores no gender at all: Poké Transporter works it out from the Attack DV at the moment of transfer, and by then the answer is locked. Type in the stats the game shows you, and this walks back to the possible DVs, and so to the gender you are about to get, while there is still something to be done about it.

The long-range trainer glitch spawns a wild encounter whose species comes from the Special stat of the last Pokémon you battled, so the species is settled well before the encounter begins. Pick the one you want: this gives you the Pokémon Red trainers that leave the right value behind, with the map, the coordinates and the party slot.