I noticed this spacing behind the stove, on the floor between wall and floor. Is this normal? Should I seal it with something?
Yes it is normal and no, you don't need to seal it. There's always a space between the floor and wall, but in finished parts of your house, there is baseboard that will cover it. If you want to spend money on baseboard behind your stove, go for it, but it serves no purpose other than aesthetics.
As Steven mentioned, it's normal. That's a gap to prevent water on the floor or in your slab or foundation walls wicking up into the (moisture-sensitive) paper-faced drywall. There's only one reason you'd want to seal it up: if it's a path for air infiltration or exfiltration through the wall. If you discover that this is the case, you can seal it with a high-quality silicone caulk. Don't use drywall filler or spackle (they'll wick water) or spray foam (it's flammable and this is behind a stove).