My misunderstanding. I thought you meant from the heater to the floor.

To fix the gap you were talking about, you could do some acrylic caulking. You could do high heat silicone if you were worried about the heat, but the acrylic should be fine. Check the tube for the allowed heat range. The silicone will do vetter against the expansion and contraction, but you’d never be able to paint it. The acrylic would be paintable. But had a high chance of cracking.