The redirect_uri_mismatch error message would be coming from Google and means the current GCP settings don’t allow the redirect you have requested.
I just gave this a go on my own side, with n8n showing a callback URL like this:
I added the full URL to the respective field in the GCP credentials settings after creating a brand-new project:
Opening the Sign in with Google link then takes me to the consent screen:
After allowing the connection I end up here:
So, if Google disallows the redirect without providing any further details you might need to reach out to their support team to learn more about why they would do so in your case.
It might also be worth to try and force a new tunnel URL by closing the desktop app, following the steps suggested here, and then restarting the app: Shopify Trigger Not working - #6 by MutedJam