After reflecting on this and from
@KontradictioN 's suggestion that Gronk may have a high ankle sprain, I'm quite confident to say at least this for certain: he is not able to jump right now. There were several times they had a spread formation near the red zone and Gronk was single covered on the outside with a massive height advantage. Normally that's an obvious fade pass with a high success rate. In addition, every ball he's caught has been thrown to hit him in stride. Normally, you'll see Brady float at least 2-3 balls to Gronkowski, even when covered, that require him to out-jump the defender. We haven't seen that since week 1.
That makes me quite confident that Gronk is being severely hampered by an injury rather than just slowing as part of the aging process. Hopefully it isn't a high ankle sprain because those can take a full season to heal. Whatever it is, though, I think defenses have caught onto it and that's why they're putting quicker, smaller players on him. His straight line speed doesn't seem to be as badly effected (example: that last catch).