If you get a lot of email, once it starts syncing it's going to keep on syncing until it's finished. If that's a few hundred emails a day, it could use a lot of battery. If that's the case, there's not much you can do, except set filters in
GMail so you aren't syncing anything but important emails. (Forward anything from senders who aren't important to another account, then delete them on this account. Read the other account from a computer.)
If you get a lot of spam, use the filters to delete the spam, so it's not there to sync. (You really don't want to make a million dollars, save on your car insurance and grow larger body parts, do you?)
Another thing you might try is to not sync anything in GMail and use a different email app. (Any email app can sync your GMail email accounts.)