So, given that the average lifespan of an NFL career is 3.3 years, one would assume that a player who was drafted in 2015 and still a starting NFL RB is at least "good".
Part of the problem is that the terms we use are very malleable. "Elite." "Great." "Top." "Good."
These are all going to mean different things to different posters.
Having watched a ton of Gordon snaps, my opinion is that he was an average player and talent at his position.
I mean, outside of an outlier year (2018), in which he was 8th in yards per carry (and in which his backup actually was slightly better on 100+ carries), he was dreadfully inefficient. Just look:
2015: 3.5 YPC, 43rd
2016: 3.9 YPC, 29th
2017: 3.9 YPC, 26th
2019: 3.8 YPC, 36th
Those are his ranks compared to other running backs (QB's excluded) to have had at least 100 carries in that particular season.
People might point to his work in the passing game, and he's a decent receiver for the position, but there too he has not been efficient very often. For example, his 7.0 yards/reception last year was 24th among running backs.
These numbers are indicative of why I say he's not been a good player, rather he's been a high volume player.