Does blocking a contact on iPhone affect Gmail emails?

If you block a contact who sends you a message on your Gmail, which is synced with Apple Mail, the email will be blocked.

The sender will not be blocked in the Gmail app. The blocking happens locally in the Apple Mail app, along with your Contacts synced in iCloud. If you want to completely block someone from sending you messages to your Gmail address, you must block the person directly in the Gmail app, or via the website.

Yes, if you have the Gmail app, blocking the contact on your iPhone will not have any effect on the emails you receive from the same contact in the Gmail app.