Collagen is marketed for hair and skin only. Does that imply that it has no use as a general protein supplement for muscle gain? If not, why?
Collagen protein, regardless of specific type, does not seem to generate a muscle protein synthesis or connective tissue protein synthesis response. This, in addition to pricing, suggest it would be a poor choice for protein supplementation for the purposes of muscle gain.
It also does not seem to do anything unique to hair, skin, or connective tissue.
For further information, you may be interested in the review podcast I did on the topic: