However, the Gmail App push notification works with wifi, even with 3G cellular data turned on. I am guessing that the Gmail App somehow knows to choose wifi over cellular data for push notifications. That's the one difference with the built-in Mail App and the Gmail App. Unfortunately, when I am on the road without wifi, Gmail push stops working. Fetch is not an option on Gmail App either.

So here's what I have done on my iphone 4s with SO UMB. Use both Apps!
(1) Configure built-in Mail App to fetch every 15 minutes
(2) Configure Gmail App to same account

Benefits:
So now when I am connected to wifi (at home, work, Shaw wifi hotspots, etc) I will get push email notifications. When I am on the road without wifi, I still get my fetch email checked every 15 minutes.

Down-side:
When on wifi, if new emails pushed by Gmail App are not checked after a certain time, the Mail App will still continue to fetch, resulting in double notification/banners/alerts. The Double banners or notification sounds for each email doesn't bother me, and I can also just customise the gmail app not to have banners/alerts and only have the notification sound, etc.

I am happy to get partial push email back. Not the most elegant solution but works for me.