Enormous mail file is taking over my hard drive storage, maybe disabling Mail search function and sync.

In Mail, click on the Window drop-down menu and select Connection Doctor. Uncheck the box "Log Connection Activity". Then quit Mail and trash the log files. They won't be recreated because logging will be turned off. It's a trouble-shooting tool you only need if you are trying to figure out why your Mail isn't connecting.

Apple Mail will by default download the entire Gmail mailbox and that sounds exactly like your problem. Here are your options:

1. Configure Gmail to limit the number of emails it delivers over IMAP by going to your Gmail web interface, clicking the Gear in the top-right then See all Settings and go to the Forwarding and POP/IMAP tab at the top. Scroll down to the IMAP section and set Limit IMAP folders to contain no more than this many messages (1,000 - 10,000).
2. Configure Mail Account settings for Gmail to not download the attachments on the Account Information tab in the Google IMAP account settings.
3. Uncheck the "Enable this account" and Quit Mail
4. Press Shift+CMD+G to pull up the Go to Folder in Finder then go to ~/Library/Mail/V9 and you should see several long numbered folders expand each one till you find the one that contains "[Gmail].mbox". You will then move that numbered folder to the Trash.
5. Start up Mail and re-enable Gmail account and let it sync.

If that doesn't free up enough space you'll either need to start using Safari or perhaps Chrome to access your Gmail instead of the Mail app. Otherwise you will need to delete your old email on Gmail. Easier if you do it on the web interface to Gmail.

Well you did such a good job communicating the problem. That was key in figuring out what was going on. Glad to have helped!