iPhone does not sync events to Google Calendar
It often takes a few minutes for the cloud to sync, that's normal.
I'd suggest double checking your phone to ensure your push settings are all set the way they ought to be. Since it's working on iPad, but not iPhone, it's my assumption something got funky in the settings and it's simply not being pushed to the cloud.
easiest fix, delete your gmail and redo it, or compare each item on your phone to your iPad.
Also, a restart my help as they get silly sometimes.