Mathis only started 1 game in his first 3 seasons combined, but still had 3.5, 10.5, and 11.5 sacks as a part-time situational player. Then, once he became a starter, he had 9.5, 7, 11.5, 9.5, 11, and 9.5 sacks in his next 6 seasons in the league (that's 83.5 sacks in 9 NFL seasons).
Williams had 4.5, 14, 12, 9, and 8.5 sacks in his first 5 seasons in the league, then had 5 sacks in his first 5 games last season before going in injured reserve (that's 53 sacks in 6 NFL seasons).
Those guys give everyone fits, not just Gilbert. If Gilbert is our LT next year, he will face elite pass rushers more often than at right tackle, but aren't many DE's in this league who are better than Mathis and Williams.