Horizon Forbidden West: Should you head west or go back east after meeting GAIA

After you've completed the first two major regions of Horizon Forbidden West, Aloy will introduce you to the new version of GAIA she's creating. This is when you'll set up a base of operations from which you'll go out and gather the AIs. However, after meeting GAIA, you must make a crucial decision: do you return east or travel west? Each choice is explained in detail in this tutorial.

Pick one and come back to the other later

In the end, it makes little difference which option you choose. The only effect it has is on how much time you have in the game once you leave the facility. You'll advance the story and enter a new section of the Forbidden West if you opt to travel west. For each of the subordinate AIs, Aloy will gain experience and unlock three new main quests. You can then choose which mission to pursue or simply explore the west at your leisure.

You'll be able to finish all of the side quests, contracts, and activities you left behind in the previous zone if you choose to return east. However, if you wish to empty out the area before moving on, you should return east before going farther west and continuing the adventure.