I am trying to open one GUI from a completely different GUI. I am developing on a desktop and the windows have different sizes from each other. I looked at screen manager but I feel as if there is an easier way to do this.
Thanks in advance!
It's possible, but kinda inconvenient. The issue is that kivy supports only one window per app, so you need to work around it somehow. I personally just use multiple *Layouts (which are different GUIs with different functions) in a single window, showing and hiding them as necessary. Obviously this approach has its restrictions, eg it doesn't support multiple monitors, but it's as simple as it gets.
Then there is a question here on SO where people spawn separate kivy apps for every window, thus getting windows that can be dragged and resized relatively. It requires some fiddling with subprocesses and communicating between apps, but this method is more powerful.
ScreenManager, as I understand, doesn't help you: it allows just to define multiple widget trees for the same window and switch between them on the fly. It's a normal use case on touchscreens, but makes pretty little sense on desktop. Which is true for quite a few things in kivy, to be honest. If you don't plan to move to mobiles later, Tkinter or PyQT may be a better choice than kivy.