Hi @karl_oliverio ,
Do you mean that you create a custom form in sharepoint list?
Firstly, it's just a form in sharepoint list, not app.
Secondly, could you describe more clearly about how do you insert a screen of gallery?
I've made a similar test, but not met the same problem like your description:
1)Inset a blank screen (not scrollable)
2)inset a gallery (gallery is scrollable by default)
You could turn off "show scrollbar".
3)adjust the gallery's size
set it full of the screen
Then, my form shows like this: