I would use your cabinet trim and also a piece of counter if you have some left.

I used these below (they're out of stock but the page leads you to others).I'd use a trim board or a piece of counter on the open side with one of these, or center the range doing similar.

https://www.amazon.com/gp/product/B01N1JNTYD/ref=ppx_yo_dt_b_search_asin_title

Thanks, Kate! Unfortunately no counter left :(

Thank you, Fori! Yea, the more I am thinking about it, the more shifting the microwave makes mores sense that way there is only the one gap to worry about between the wall and stove. SS is probably the move. What do you mean by "go up the wall to backsplash height"? All the way up to the bottom of the cabinet?

I think I would go up to the bottom of the cabinet, but I am kind of messy when I cook. :)

Is your backsplash actual wood or wood tile? If it's tile (I'd hope so, behind a range like that), you might be able to use it for the gap--actually, if you have more available, then I'd want it on the side to make it look more finished regardless of how you fill the gap.

What kind of granite do you have? I'd call some fabricators and try to find a piece of 4" backsplash remnant, then beg one to cut it to fill the gap to the left (after moving both the range and MW to the right. It can be supported in one of several ways but getting the piece of granite is the priority. The fabricator might even be willing to cut a narrower strip for the back.

ETA, I had a local monument/countertop company cut a couple of paving stones (maybe blue stone?) I found elsewhere. I just had to wait until a day they were between cutting other stone. I think each cut was $5.00.

So - shockinly enough, it is actually wood. It does have an epoxy-like coating so it wipes up no issue. It spans the length of the kitchen, behind the sink and the rest of counters/fridge. Quite odd. I think they were trying to match the island, which is beautifully done with that same wood finish (as well as shelving throughout the house), but I don't know that I personally would have chose wood as the backsplash. I figured i'd get the cabinets done and then see if I want to keep the wood backsplash.

A wood backsplash behind a slide-in range would worry me. No large-diameter hot pans on the back burners, at least. Building on Fori's idea, what if you installed a stainless backsplash and sidesplash up the height of the upper cabinets, and a narrow stainless countertop piece between the range and the wall? I don't know whether you could cover the wood with stainless, or whether you would have to cut it out from that location. Other experts here could advise.

Yeah, I never really put too much thought into the wood behind the stove. But, you make some great points. For me, I'm not a big cooker. So, it's never been an issue. But, this house will be a rental in the coming years, and probably best to cut the wood out and do stainless or something more durable to heat. i think i'll actually be able to use that wood to fill in where i'm taking out cabinets above the fridge.

I'm going to frame in that side next to the wall, move the microwave over and will keep my fingers crossed that I can find a peice of granite that matches the rest well enough! If not, then i'll just use some stainless steel that feeds down from the backsplash.