According to The Book of Geese and Storey's Guide to Raising Ducks, goslings 4-12 weeks should be on 16-18% protein, and 13-26 weeks 16% protein. If they have access to pasture, feed as much as they can eat in 3-5 minutes once daily, if no pasture, feed that amount 2x daily. For ducklings 2-7 weeks 16-18% protein, 7-20 weeks 15-16%. You can add rolled oats, or greens to "dilute" the protein. Recommends free choice for 5 minutes 2x daily for ducklings.
Flock Raiser is 20% protein. Start and Grow is 18% protein. My local feed store carries a "poultry developer" that is 17% protein and "supplemented" with niacin, I have been adding Brewer's yeast as well. My Pilgrim gander seemed to be growing too big for his legs for a while, he just didn't have any stamina and one of my ducklings wings started to droop a little, so I freaked out and stopped the Flock Raiser and limited the feedings to twice daily once they were 3 1/2 weeks old. Seems to be helping. They are still growing like weeds, but the wings look okay and the little gander is much stronger/more balanced.