Discussing conditions of an offer
For minimum GPA, there isn't much utility in letting them know ahead of time.

They either have an absolute GPA cutoff or they have some flexibility. If there's an absolute cutoff, then there's not much you can do.

If there's flexibility, then you can try to argue mitigating variables at the point when you have to tell them your final grades. There's not much advantage in starting the argument before you present your grades, and in some ways this may hurt your chances because they might think you should be using your energies right now to try to improve your grades.

In all likelihood, you were accepted with awareness that your grades were weak. In that case, they may want you to go on academic probation or you might have to forgo some scholarships/funding until your grades improve. Not the end of the world.