How to login to Pokemon Go with different Gmail account than in Oreo phone? I have a different Gmail account connected to my Pokemon Go than the gmail account in my phone. 
The game attempts to use phone's gmail account, and it does not allow to set login details manually.

How thinking how to troubleshoot?
Niantic's support forum is down, and I cannot ask the question there. 

Phone: OnePlus 3T
OS: correspondent to Oreo
You cannot. This is a "design bug" in the game system.