Pokemon-go – Why can’t I log back in to the Training Club Account on Pokemon Go

You have to go into Google account and remove the device you're playing on. Then sign back in and reopen app... I literally messed around for 14 hours and missed so many at Disney because of this and it worked the first time, same problem, same phone...

As far as I know (I don't have a source other than personal observations), Pokemon GO doesn't access your Google account at all.

When you "log in with Google", the game checks whether a Google account is present on the phone and what the associated e-mail address is, then loads the Pokemon GO account identified by this e-mail address, without ever trying to access or otherwise use the Google account. It has a completely seperate account based on your Google account's e-mail address, which is the only thing it needs to load that account, operating under the assumption that the Google account would only be present on the phone if you had the password.

Therefore, Pokemon GO will never show up as a connected app in your Google settings. You can't revoke access because all it ever needs is the name of the Google account linked to the device and you really can't hide that from an app. (Also why would you?)

This also means that should Niantic ever shut down their servers, everything Pokemon GO related will be gone, there will be no way to get any records pulled out from your Google account. If someone subsequently created a fanmade clone of Pokemon GO, everyone would have to start over from scratch, it would be impossible to pull any account info from Google or Trainer Club because there's nothing there. This is notably different with games that actually connect to Google, those store settings and usually progress in the Google account and if they discontinue the service, someone can just take over.