Trying to login to Pokémon Go with Google and I get a Error 403 code?
There is a Pokemon support article on this:

These error messages may indicate a temporary issue with the servers.
First, close and then reopen the Pokémon GO app:
iPhone X or later: Swipe up from the bottom of the screen and pause in the middle of the screen to view the most recently used apps. Swipe left or right to locate the Pokémon GO app, and then swipe up on the Pokémon GO app to close.
iPhone 8 or earlier: Double-tap the Home button to view the most recently used apps, swipe left or right to locate the Pokémon GO app, and then swipe up on the Pokémon GO app to close.
If that doesn’t resolve the issue, please visit the Niantic support page at https://niantic.helpshift.com/a/pokemon-go/?p=web for more information.
Note: The link takes you to a website that is not created or maintained by The Pokémon Company International.