If you are getting Gmail FCs, before you do something as drastic as a factory reset, I'd try this:
- from home, settings->applications->manage applications
- Gmail should be in the "Downloaded" tab. (If not, it will be in the "All" tab.) Scroll down to it and tap it.
- Try tapping "clear cache". Go back and se if you can get into Gmail now.
- If clear cache does not help, try "clear data". This will wipe all of gmail and settings off the phone, so you'll have to go back in, wait for it to sync again, and change any settings you may have made before (i.e., labels to sync, notifications, etc.)
See if that fixes it.