GTA Online Gunrunning update hints an entirely new UFO mission that is scheduled to unlock in future this year. It was noticed in the update where you will find a crashed alien ship near the Zancudo Military base in the game according to a Youtube video by Chiliad Mystery Guru.
This looks a secret mission hidden in this open world game, this was discovered in the video below and played by the uploader. You can see the player reaches near the crashed UFO and shoots aliens around it. The mission does not seem to be completely unlocked except a few aliens attacking.
The crash site has dead people around who seems to be a part of research team, there are also alien eggs and many vehicles. Rockstar looks planning to release an alien-theme base mission in future. This is the first time aliens are seen in the game. Independence Day-themed event has also started today and will run till July 10, where you get big discounts on America-themed items, vehicles and weapons. There are also firework launchers, monster trucks, and outfits.