Everything I tried left a fairly defined horizon line, except for one:
Use the same gradient for the floor as you did for the back, and reverse it's order so that the two edges match at the same color (you can manipulate the floor gradient afterwards to change in the direction of the viewer.)
Easiest thing to do is create the back wall; create the gradient effect. Set the Anchor Point at the bottom edge. Duplicate the wall. Rotate the floor 90 degrees on X. Position in space; scale accordingly.
Go back into the floor shape gradient and change as needed – anything but the edge color that contacts the back.
You can't use a light. So fake one. You can add an oval shape behind the main subject - large feather value - a color blend mode like Add (turn down opacity to taste.) You can use a Reflection on the floor.
The effect will be seemless... but also a lot more work achieving a balance.