Many companiese offer a "balancer" mineral for use with gluten feed and DDGS. Some will be just minerals, whereas some will be mixed/pelleted with a protein source as a carrier.
IMHO, the mineral only product is the way to go. You've already got the protein, why spend more for stuff you don't need? Also, you use less product per ton of mixed feed; mineral only will be fed at 0.33 lbs/hd/day or so, whereas the balancer+protein will be at 0.5 - 1 lb/hd/day.
The balancer should have primarily calcium in it, with salt, potassium, magnesium, and trace minerals. Pretty basic stuff. If it has ANY phosphorus in it, you're double-buying it and don't need it.
Watch these feed companies - they want to move their high priced protein one way or another, and slipping it into a balancer is one way they're trying to do so.