I go to my local supermarket and just buy one scallop at a time, cut it up into mouth sized chunks (gigs mouth size that is), put it into a ziploc and soak some selcon in then. Put it in the freezer, and break off a piece, defrost, and Gigs love it.
Be careful not to feed your Gig too early... take your time and let them acclimate, generally a couple of weeks at least.