I can see 2 reasons why Gibbs isn't mentioned as one of the greatest NFL coaches ever.
1) He is a reserved personality. Never generated a lot of attention for himself on the national scene. Was always happy to let everyone else in the org get attention. Meekly loved the Lord, etc.
2) He might have been a victim of what made him one of the best. After Joe T., none of his quarterbacks were particularly engaging on national scene. He won with a lot of quiet, no-name type players. The Hogs remain the most enduring personalities at least tangentially related to the org.
Bonus:
3) Kind of relates to #1, Joe took care of his business in-house. If he had a problem with a player, or a coach, or was disappointed in the team's performance in general, he kept any fiery or spicy talk behind closed doors, away from the media. He rarely gave the media much to blow up into big stories.