Random [Google Mail] folder/label in iOS Mail after updating to 10.3
I had the exact same issue and found this solution online. It's working for me and the [Gmail] folder is no longer there. I think you would just substitute [Google Mail] for [Gmail]. Good luck!

"That folder is where gmail store all your mail (like the enclosing folder), I think it's a weird artifact of the way that Google have designed their IMAP system.

In Apple Mail, open the Accounts preferences, select the Gmail account > Advanced Tab. In the 'IMAP Path Prefix' insert [Gmail] (or however the folder is named - for me it is [Google Mail])."