Oddly enough, the Stock Items screen and GI do not exist on the mobile app by default.  Adding a basic screen to the Mobile app is pretty easy once you learn how, so I recommend completing the 2 free courses T400 and T410 related to the mobile app.

https://openuni.acumatica.com/courses/development/t400-customization-of-mobile-application-basic/

This will tell you everything you need to know for adding a GI to the mobile app… and then some!

If you don’t need anything fancy, there is a checkbox on the GI to expose to mobile, and it will simply drop the GI on the mobile app for you.  However, I still highly recommend the free training courses from Acumatica for anyone wanting to work with configuring the mobile app.  It will show you how to customize what fields are displayed, customize the layout of the screen, and add actions and some pretty cool built-in features like being able to capture a signature.