Who's arguing for Tony Gonzalez to be 99?
A 99 player should execute every aspect of his position at the highest level. Gates is probably the best receiving TE going today, he doesn't execute every aspect of the TE position at the highest level and that's not really in question.
If this debate were taking place 15 years ago, I would argue against Deion Sanders being a 99 overall by virtue of his tackling, although he's definitely be 99 worth as far as man coverage, speed agility, etc.