Great Dane, Raw Diet, Weight gain supplement?

If the dog is healthy, I recommend simply adding calories.  For a dog like a Dane, also make sure the dog is active enough to be interested in food.

When working with straight bones/meat diet, some dogs don't really have the physical space or metabolism to be satisfied with only animal-sourced nutrition.  Dogs at the extremes often fall into this class.   Pointing to models from Mother Nature for nutrition for such dogs, is irrelevant.  Mother Nature would have culled 125 pound Great Danes - huge dogs, giant metabolisms, and relatively small organ space - in her quest for efficiency and moderation. 

So carbs are an easy and safe way to work in more calories.  But then you are leaving the safety of the world of relatively appropriate nutrition - meat's got at least a decent profile of minerals for dogs - not perfect because we don't feed stuff like eyeballs, hair, reproductive organs, ears etc or not in the right proportion.

So then - particularly  for an "extreme" dog like a Dane, you'll want a diet plan that balances the reduction in mineral content, with extra supplements.

I can't just tell you what supplements those would be.  It varies wildly depending on the meats, offal, and bones used for the base of the diet. 

Lack of some of these minerals can make a huge difference in weight gain and appetite and muscle condition in a young dog.  I had my diet evaluated for my high performance young Border Collie, Ted.  I've always had trouble getting Ted up to good condition.  Turned out he was low in potassium.  Low potassium will prevent a dog from building muscle.  Adjusted that and he started gaining weight immediately.  I was even able to reduce his calorie intake a bit.