1. Gmail does not have push (as others have stated).
So if Gmail is your only email, just set it for manual or set it to fetch every 30 minutes or 1 hr if you want your email in realtive time.
If you have MS exchange or Mobile Me or Yahoo, than you qualify for "push"
In relation to these push emails, battery life depends on how much emails you are getting.
If you only get 10-15 emails a day, than push email is much better on your battery life than trying to fetch your email every 15 minutes.
Now if you get 100 plus emails a day, than push emails will absolutely kill your battery.
Push leaves a constant data connection so the more data (push emails going through) the more the battery drain.