Find the default gateway IP address of your router using your iPad/iPhone (iOS) is quite easy:
There are many network scanning apps for iOS. Just search for "network" in the app store an look for an app which can scan your network. Just about any of them will do, so I won't recommend any because they often change features and price. Once you start your scanning app be aware that it might not identify your gateway specifically. Most apps will simply show all the devices on the network.
You need to look for the router by name and then you will see an ip for that device. This is your gateway ip (the IP address of your DEFAULT gateway).