Kirkaland premium lamb and rice, the green bag. Two cups, twice a day maintains my GSP's weight with regular exercise.
I free-feed him during hunting season, he loses 5-8 lbs during a 3-5 day hunting trip. More if the hunting is good, or warm.
I also feed him the same brand of wet food during the trips. He will always eat it, not so much with the dry.
Seems like most of the season is spent trying to keep weight on him, time off is spent recouping and healing.