Hi Samer,
you might have resolved this by now (sorry - only just read this thread). Anyway, the issue of adjusting the input gain shouldn’t really apply via an external audio interface like one of the Focusrite Scarlett models as these units feature hardware controls for setting the input gain (I use a Scarlett 8i6 with my iPad/Cubasis and it works well).
The original poster’s problem was with USB mics like the Samson Meteor that connect via the Apple Camera Connection kit. These mics often don’t have an input gain control to allow adjustment - hence the problem if you need to adjust the gain. BTW, I’ve also got one of the samson mics but have not, at yet, tried to use it with Cubasis… Off to experiment now… 
Best wishes,
John