With Hide 'Delete' Quick Action for GMail, you can choose to hide the email's Delete option within the GMail interface. It is a small program, and all you need to do is to install it, and the result should be displayed on the menu bar. With this tool, you can also hide other menu options on your phone, like Backup or Archive.

The program does not require any kind of root access, and you won't have to add your phone number for the utility to work.