imap from gmail to apple mail

You can if you want, but there should be little need to.

IMAP accounts store message data on the server, so you can access the account from multiple devices and get a consistent view of the mailbox (including status such as read/unread status, etc.)

The only reason to manually synch would be if the local system was out of sync for some reasons (e.g. your device was offline for some time and hasn't (yet) recognized recent changes to the mailbox), but even then it should eventually catch up. Forcing a sync is just a shortcut to force what would happen normally.