It's absolutely normal that vehicles that are generated by the sim automatically ( which are default Ground services with their models replaced by GSX, but are not handled by GSX ), would disappear and replaced by real GSX vehicles on the FIRST operation you made on a gate, because that's what activates GSX.
So, assuming "Operate Stairs" was in fact the first thing you did from the GSX menu on that session, that's exactly what you are supposed to see.
What is not normal, of course (assuming that with "I was trying to fly on oslo", you meant you were parked on a gate AT Oslo, not flying TO Oslo ), is seeing China Eastern airlines vehicles there.
Now, you said you saw the *stairs* changed to China Eastern, and you need to be a bit more precise here because, if the stairs were changed to China Eastern, but other vehicles had different operators, is ONE kind of a problem which requires a completely different solution than if ALL GSX vehicles that appeared were ALL China Eastern.
