I have had my iphone for a over a year, but in the last couple of months I am not able to get gmail unless I am in a wifi area. I see data bars on the phone.
Are you able to use cellular data for other things except for email?
Check and see if in Settings under Cellular if the button for your email is green (toggled on). If not, toggle it on.
Here are a few things to try if it is already on:
1) toggle Airplane Mode on and then off again and see if that helps
2) do a service update by dialing ##873283# and pressing the Call button and then waiting for the update to complete.