How do I shield my microwave from my gas stove?

Is your microwave on a closed shelf above the stove, or how is it mounted on the wall? Posting a photo would help us to understand the situation.

If it's just temporary, you can make a heat shield by bending a piece of sheet metal to fit the area. Then insert a piece of closed cell sheet foam between the microwave and the metal shield. It won't be pretty, but it should do the trick and be removable after cooking

If you want a non-conducting heat shield, I would cut a piece of cementicious fiber board, or tile backer board, aka Hardi Backer, to fit. It must be cut outside, due to the fiberglass.

I would paint it, to seal the edges.