How can I remove this site from appearing on search suggestion?

It’s probably what is called an auto-complete suggestion. The blue-text link in my reply above describes how to remove those. Tap or click on the blue-text URL and read how to remove the suggestion.

But it’s easier to add gmail as a favorite though, then tap on it. That avoids typing in anything; ‘gm” or otherwise. That avoids this and other problems with the approach that you’re using here.

Or add your gmail account to Apple Mail and use Apple Mail, and avoid using the browser entirely.

Three possible options:

Add gmail to your favorites, and tap or click there; on the favorite.

Or add gmail into Apple mail, and use Apple mail to access your gmail.

As for your question: https://apple.stackexchange.com/questions/122428/how-can-i-remove-an-autocomplete-suggestion-from-safaris-address-bar