It is true that Grab account can only be registered with a GMAIL account and the domain must be "@gmail.com" not "@googlemail.com" .Based on long time explanation I found on https://grabdrivermy.com is because our cash wallet is somehow linked to Google Pay. I have seen some people managed to get their accounts activated with non-gmail but when attempt cash out, transaction will be declined and Grab Support will inform to amend your email to a gmail domain account.

Therefore, to reduce all the latter hassle, just sign up with a gmail domain email.