What is the appropriate way to share a subcomposition?

I have a few subcompositions that I’ve created and added to my own node library. If I’m interested in sharing some of these, along with example compositions, is it appropriate to send them along as potential additions to the Node Gallery, or is that space reserved for nodes created using the SDK?

Sure, subcompositions could go in the Node Gallery. The Node Gallery is for generally useful building blocks, and there’s no reason those couldn’t be implemented as subcompositions.