How can I prevent objects from falling into the gap behind my freestanding range?
Normally there are two options, though it sounds like you just replaced the range and haven't made any other changes, so you might only have one.

If its the plug sticking out that is the issue, then either yourself or an electrician (depending on your skill level) can change out the outlet for a recessed outlet. Whether you can do this will depend on a lot of things, like what is behind or in the wall.

The other option that is more common when installing new counters is to cut a thin piece of the counter top to go behind the stove. You would also need to support the piece, and again, the type of material will dictate how much support is needed; laminate is a lot lighter than granite, for example.