Saw this on their website. Excludes the $20 card. Expires on 29/06.
Hope it helps.
edit: Use AmEx Statement Credits for a further $5 off if you spend more than $15 and pay with your AmEx card. TOS say nothing about excluding gift cards. Link to deal
With 20% off at OW & Target this doesn't really cut it as a deal, IMHO.
Unless you can't get to those places.