On my Mossberg 9200 the vent rib on the barrel is loose. Is there a way to fix this or can a gunsmith even fix it??

A gunsmith might be able to fix it. Or you might have to send it back to mossberg and they'll fix it. A friend of mine had the part of the barrel that attaches to the foreend of his 500 break off and Mossberg fixed it for him, if I remember right, they did it for free or for a low cost.

I would check with your local gunsmith and if they wont or dont feel really confident in doing it send it to mossberg.