This menu is completely controlled by Google inside of the gmail app. We are unable to change this menu, as it is inside of a non-Brave application. You will need to contact Google with a feature request.
Apple does not allow default apps to be changed. If you would like to be able to use Brave as a default iOS browser you will need to contact Apple and request this as a feature.
Both are largely unfortunate for us, but there is nothing we can do at this time. You can always attempt to long-press a link and select the “Open in Brave” option inside of the action menu.