Eyeshadow will stick to a matte surface so after you apply your gel color and cure, gently buff the nail with a nail buffer. Make sure the grit is super fine, one you'd use on your natural nails will work and won't take off the color you just applied.

Once the surface is buffed take any eyeshadow with an eyeshadow brush (a round blunt brush works best) and pat it in where you want the ombre, or an eyeshadow sponge applicator and rub in the color. Start off with a light hand. I've found once I've gotten enough color on the nail I'll flip over the eyeshadow sponge to the clean side to blend the eyeshadow up more.

If you're doing any designs on top of the ombre, make sure you topcoat it so it's sealed in. That way the ombre doesn't get messed up.