Clear Backing ~ shown in the pic on a light nude base colour.
Supplied in a grip seal bag
Bellflower Transfer Foil
Transfer Foils need a sticky base to be applied to. If your gel polish as good sticky dispersion/tacky layer then you can dab the foil into this layer.
If you need a more sticky base I can recommend Nail Foil Adhesive or the Stick It Collection.
Stick It in the pot is good for opaque/thicker foils
Quick Stick It in the bottle is good for thinner style foils.