Hi @ConfusedAnt, so I gave this a go on my end and could fetch my media items using the GET https://photoslibrary.googleapis.com/v1/mediaItems endpoint:
Example Workflow
To set up the credentials, I have used these settings in n8n:
I noticed you have a different access token URL - I am using https://oauth2.googleapis.com/token found here.
I’ve also added these scopes: https://www.googleapis.com/auth/photoslibrary https://www.googleapis.com/auth/photoslibrary.sharing https://www.googleapis.com/auth/photoslibrary.edit.appcreateddata and access_type=offline as the query parameter.
Maybe you can give this a go on your end and see if any of these does the trick?