I've used it before, its a vitamin supplement, and wont put any weight onto your dog. I find the best thing for weight gain was Lamb, but even then that wasnt the only thing they were eating as I feed them Chicken (legs, wings, thighs and necks) minced beef, beef heart, and anything we catch as well.
Some dogs don't like having SA37 sprinkled onto their food at all and will not eat their snap if they smell it in there, so with your dog being a fussy eater you'll probably bump into some problems with the powdered gear. I used to put it in with Minced beef and give it a good mix and had bet