Does online gaming use data?
Go to Settings > Cellular and scroll down. There should be a breakdown of Cellular Data used by app.

If you didn't know this option was there, then you likely will not have reset it, so it will represent all of the cellular data you have used from when you first added your id to the device. 

That being said it should give you an idea of what apps are the big cellular data users.

It also gives you the option to switch off cellular data usage on a per app basis, so that you can control which apps get to use the cellular data stream and which have to wait until your device is connected to Wi-Fi.