I have not tried with the Gemini but I think a lot depends on the type of scanner? As an example
At Amsterdam check in gates the camera is on a bracket some 30 cm above the desk and you just put your phone under - I keep holding the phone so the distance varies a bit, the code on the KLM app is read fine - I think the Gemini would be OK
At Glasgow you have to place the phone flat on a plane of glass in the reader - the camera is below the plane of glass, I think in this case it might be harder due to the keyboard.
You could always give it a try and have a paper copy just in case...