How big of a gap? Do you have a photo? If the gap is small painters caulk is best and then paint and it disappears.

You can do full height solid surface instead of just a 4" backsplash. Or a large format tile so there are very few grout lines. Lots of options depending on the gap you need to fill.

Do a 2” splash. Or back painted glass.

concur with Cook'sKitchen. A short, 1 or 2 inch short splash out of the countertop material hides a lot of sins and is much less obtrusive than the taller 4 inch version. Paint the rest of the splash area a close color and you won't even see the short splash.

Check out this discussion to see if any of the suggested options may fit with what you want. https://www.houzz.com/discussions/gap-between-backsplash-and-granite-dsvw-vd~2993575

Just have in mind that backsplash tiles should not actually touch the countertop. Wall tiles should be installed approximately 1/8 inch above the countertop. This is an expansion joint that is necessary because a house may settle or move slightly over time and make cabinets or countertops put stress against backsplash tiles. This gap between backsplash and countertop will prevent any damage.