Is there a way to transfer a boarding pass from Gmail to Wallet on iPhone
Unfortunately, it's up to the airline to handle that. If there is a pass available for you Wallet, there will be an icon on the website where you can print the pass that says Add to Apple Wallet/Passbook

If not, you may have to download the airline's app. Even then, there's still the chance that there won't be a mobile boarding pass.

I know that with Southwest Airlines, you can only get a mobile boarding pass through their app, and only if it is a single ticket trip (i.e. no other people flying on the same purchase).