For vertical ombre I just use the brushes in the gel polish bottle and wipe them afterwards to not mix the colors in the bottle.
Foe the ombre that goes from free edge and up, it depends on the color itself. Neon type colors are easier. But white is tricky, it doesn't look good with sponge. But if you use a brand with a variety of thick base coats with different colors then it can be achieved in a similar way to how you'd sculpt an ombre. Using a milky white at the tip, blending up, the I use "structure base frosted pink" (a gel polish base) over the whole nail, and blending carefully at the tip. Then top coat. I use Lilly Nails.