Is there a way to stop a tweening Gui!?

No there's is no function to stop a tweening a Gui if you're using TweenPosition, however, if you're using TweenService then you can.

I am not sure if you're trying to just stop it while it is tweening for no reason or you want to stop it to make the same object tween to a different place.

If the second case is the case then a solution to that is that if you're using tweenposition, the fifth argument that it takes is a bool called "override" which basically if set to true means that when you call this tween, it will interrupt whatever other tween is going on to the same GUI object(if there is one) and make it tween to the last called position.

If that's not the case and you just want to stop the tween then I suggest you use TweenService.

Hope I helped, please let me know if you have further questions.