Deleting gmail on iPad inbox

For Gmail, to actually delete email instead or archiving, you must explicitly set the default action:

Settings > Mail > Accounts > [Your Gmail account] > [Account] > Advanced > [Move Discarded Messages Into] - select Deleted Mailbox

Also ensure that Mailbox Behaviours Deleted Mailbox is set to Trash.

Try the instructions in this guide published by Apple 👉 Delete emails on your iPhone or iPad - Apple Support

See whether that helps.

Axel F.