To provide some further context, the phrase film theory is quite common.
Film theory is an academic discipline that aims to explore the essence of the cinema and provides conceptual frameworks for understanding film's relationship to reality, the other arts, individual viewers, and society at large. Film theory is not to be confused with general film criticism, though there can be some crossover between the two disciplines.
RPG theory is pretty much exactly the same thing, but with a different media the target of scrutiny. And of course, it doesn't have as much academic grounding at this point in history -- we can hope that changes over time!
The concept of RPG theory exists completely separate from any particular model, such as GNS.