There is a very rare chance of a G6 reoccurring after a long period. In 17000 surgeries on G6 patients not one case of matastisis was found. G6's rarely matastisize, but they can grow large enough to affect local organs such as the bladder and rectum as the tumor grows and is not treated. Since you don't have a growing tumor this would be almost impossible in your case. Many oncologists like Scholz and Strum believe that G6 stays a G6 and doesn't morph into a higher grade and that any future higher grade tumors are different tumors from the original G6.
A reoccurrance after a long period would be extremely rare, but not impossible. 10% of all reoccurrances after surgery occurr after 10 years, but all of these are in the higher gleason grades not in true G6s.
Your chances of getting another unrelated cancer or a heart attack are far greater than your chance of a reoccurrance.