Use a matching base colour where possible.
Supplied in a grip seal bag
Abalone Shell 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.