They are not approved and designed for counter top use.
Just buy another microwave.

Gas or electric?
No to gas as the heat is intense.

I would think the opposite is more of a concern. That is, a regular microwave design does not expect to be mounted over a range top.
If your over the range microwave functioned fine over an unused range, I think it would be fine on any tepid surface. Your only concern really is that the vent is not blocked.

Can you look at the owners manual and see how it is supposed to be vented?

We had a really great ge profile over stove microwave that worked great. Just redid our kitchen and replaced with a shelf microwave. Can I put the GE one in the basement to use for heating food downstairs, or is there a safety hazard with using an over range microwave as a counter top microwave? It would be handy to have a microwave in the downstairs for when the teens are hanging out down there or I’m in the home office downstairs.
Thank you!