Many adult orthodontic patients will have some form of periodontal disease. That simply means that the teeth are not anchored to the bone as strong as one would like them to be.
But having periodontal disease will not stop you from getting braces as long as your periodontal disease is fully controlled by your dentist or periodontist. This means you are getting regular treatment and cleanings. Once you get clearance from your dentist or periodontist, then you can begin orthodontic treatment.