Prazi-pro is my prefered wormer but alot of people also use Metro.. You're looking for an actual wormer for internal parasites. as to your other quote. the big reason goldfish and other caprids are bad choices for feeders for other fish, particularly cichlids is they cause thiamine defficency when given exclusively ( well there is a link to it and Head and Lateral Line Erosion, personally I think it's poor nutrition and nutrients in general. Oscars are notoriouse for getting HLLE when fed exclusively feeder goldfish.) Gar being Piscivores ( fish eaters) in their natural setting seem to not be effected or as effected by the thiamine defficency issues as other fish in general ( not an excuse to poorly feed or care for your feeders, still bacterial/parasite potentials with poorly kept food, and a majority of those I know who feed live feed the food a good diet so this may be all the difference as often those who have HLLE issues attributed to their fish often poorly keep their feeders to begin with) Gar seem to be the exception to the general rule reguarding thiamine, you'll likely find more info on it in the Gar sub-forum by useing the search function. is it safer to get it on frozen or pellets?... absolutely.. but starving a fish indeffinately hopeing it will finally eat something imho is flat out stupid. The gar sub-forum also has a sticky on transitioning gar onto pellets/dead you may want to read.
& what do you mean by "there is little to no evidence suggesting Gar suseptible to the same issues you see in other NON-piscivore species"