Unable to retrieve node content with drupalgap
Ok, so with thanks to the advice about it being another module interfering with the retrieval of JSON data, I've identified the culprit through a process of elimination.

When I disable the "Node Registration" module the problem goes away and I can view node content through the mobile app.

Of course, that gives me another problem with how to get the functionality Node Registration was giving me, but I'll save that one for now...