The GigSky app has built in technology to recognize the device being used. The app on the dual SIM iPhones, only allow you to manage the service for the phone.
The app on the iPad will recognize the capabilities of the iPad. For, legacy iPads that do not use the Apple SIM, you will be able to manage all SIMs. For iPads with either the physical Apple SIM or embedded Apple SIM, you have to go to Settings and add data plan.
Alternatively, you can log into your GigSky account at app.gigsky.com and manage all your GigSky physical SIMs.