Your question does not make any sense.
iTunes accounts cannot be attached to gmail accounts. It's the other way around. The Gmail email address is used on the iTunes / Apple account as its primary email address.
If you have access to the Apple account, you can change the gmail address to any other address you want.
click here ā Change your Apple Account primary email address - Apple Support
If you do have access to the Apple account, you can add a payment method to it directly and upgrade the iCloud Storage.
click here ā Buy iCloud+ or upgrade your current storage plan - Apple Support
and here ā Add a payment method to your Apple Account - Apple Support
If you don't have access to the Apple Account any more, you will have to request access to it or simply abandon it You cannot change the email address linked to it if you can't access it, and you won't one able to use that gmail email address with another Apple Account either.
click here ā How to request access to a deceased family memberās Apple Account - Apple Support