After Team Guru found out about the existence of Aliens in GTA Online and then forced the mission to confirm it, they were then looking for a legit way to start the mission and after a lot of grinding, they finally have.
Guru Tadd talks about the “no shortcuts, no cheating, legit way”, he outlines the correct method to spawn the alien mission as Rockstar originally intended. He credits Guru Mama Kaimeera for the find—who’s “been grinding out the sales and gunrunning missions since [the update] came out”—as well as Guru Gramz, Guru Jared and Guru CME for their hard work.
Tadd posted on Reddit detailing the requirements, saying “As you may know from before, we changed a global variable to 20 to forcibly trigger this mission. But we have now traced back to the source, the various requirements needed. It appears that (brace yourself) the player must have: 601 completed gunrunning sales (the check is for 600 but 601 to be safe). And then start a supply run between: 21:00hrs and 23:00hrs.”
Yes, the requirements to trigger it is to complete 600 gunrunning sales( which will roughly take 50 hours in real life!) and then to start a supply run between 9PM-11PM in the game. A very highly specific time that even if someone did get to that sales, could miss out on the mission because of doing it at the wrong time.
He further states, “And to top it all off, this mission is a one time thing!! Once triggered it won’t happen again, so make the most of it lol or just watch our video and save yourself the time haha”
Turns out, it is just a one-time thing which is also quite a bummer but hey, at least we know how to trigger it in a legit way and what conditions need to be met to trigger it.