The Gmail app uses what we call server push - your gmail will be pushed to your phone so it's basically getting you your email in real time (as much as your network allows).
I don't use the Gmail app at all - I strictly use the Mail app, and set it to poll my various accounts at independent rates, and also by time of day.
My work email - every 15 minutes during work hours is fine, otherwise, hush. My friends and family sending me the latest jokes - every so often during work hours, every hour during my off-time.
And - the Mail app is HTC-specific, so it takes advantage of text reflowing on zoom, etc, etc.