Topics can frequently fall under multiple sites. For example, many proposal sites would be on topic for StaockOverflow but break off in to their own more specific communities. I don't think people working in gamification are always UX people either. In fact, most of the people we talk to about it focus more on business processes, ROI, and analytics. The UX portion is an important element, to be sure, but it's really just the most outwardly visible piece. I think the real question is if this topic is big enough to be its own proposal, and personally, I think it is.
My main concern is more about whether the questions will become too subjective.
I feel like the UX SE is more focused on the details of the user experience - the top tags there are website desgin, forms, GUI design, navigation, mobile, etc. Gamification overlaps with UX, absolutely, but it is not a subset of UX. It goes beyond the user experience in web apps and starts to encompass behavioral psychology, business strategy, HR issues, employee performance, user retention, etc.
I don't see it as the place to ask questions about what strategies to consider when trying to boost dials made by lead gen reps or if monetary rewards end up being detrimental to a customer support teams' goals. It's not the place to ask what low cost gamification techniques can be applied to a workforce that lacks access to a computer. Is gamification about making things fun or about driving business goals? How valuable is recognition when it is granted automatically through a system rather than by peers or managers? What are the motivators that drive behavior and which is most powerful?
This is a very broad topic, of which the way to display the outputs is a narrow piece. Most of the experts in the space have backgrounds in game design, psychology, and community management. Most of these people could happily answer gamification questions but wouldn't be able to address any of the other questions on the UX site with any kind of authority. They can talk about the merits of a point based system but probably don't know the answer to "What is best practice for designing form error messages?" or "Which comment sorting order makes more sense on blogs?"
I just don't feel like the topic can be fully explored if it's limited to the UX portion. There are currently only 17 questions regarding gamification on the UX site. Salesforce recently had it's own proposal move in to the beta stage and StackOverflow had nearly 1700 questions tagged for salesforce. It's been a great way to get very specific answers from knowledgeable experts and has attracted a number of Salesforce MVPs.
That's what I was looking to accomplish when I started this proposal and I don't think it's possible if it lives under the UX umbrella.