You call her a big gentle giant. Is she a draft or part draft?
Is she more TB in type?
I would feed a Thoroughbred or 'TB type' horse completely different than a draft or a draft cross. They have totally different kinds of metabolisms.
I have rehabilitated several emaciated horses and they have gained 2 to 3 pounds a day on dry feed and even more on good pasture. It is really a simple matter of more calories. The kind of horse you have determines the safest way to add those extra calories.
You can add fat to any kind of horse's dies, but those are 'empty' calories with no other nutritive value.
If you are wanting to build red meat (muscle), you have to have enough protein in the diet to supply those essential amino acids needed. This is best done with a pound or so of soybean meal. Most horses can tolerate this just fine. The exception would be a horse with limited kidney function like you see in some 'Senior" horses.
Calories can be added with grains, but this is what is tricky for horses with the kind of breeding that does not tolerate sugar or starch in their diets. TBs and TB types can usually eat large amounts grains with no problems at all. Many race horses eat 25 pounds of grain a day (some eat more) while they are in race training. Many drafts and some other horses can only sniff grain and have problems.
So, one thing is for sure. Your horse is not ingesting enough calories. We need more information to give any advise on what those calories should come from.
I hope this all makes some sense. People new to horse ownership can be overwhelmed by all of the details they never knew about.