For starters, we can't go to "if-he-only-played-on-the-90's-Cowboys" ville. We just can't. That could probably be said about any player that played at the time. Just like we can't say that "if-Gore-only-played-on the-Patriots-he-would-have-won-3-Super-Bowls . . . because he didn't. In fact, the combined record of Gore's teams has been 116-118-1 so far. If the Bills go 4-1 the rest of the way and he retires, he would end up on teams that went 120-119-1 in the regular season.
Maybe a better analogy for Gore would be to use baseball. Let's say Gore put up a year with a .280 average with 160 hits, 22 home runs, and 85 RBI as an outfielder. Not bad, not great, and probably above average. But multiply that times 20 years and now that player would have 3,000 hits, 400 home runs, and 1700 RBI to go along with the same .280 average. Those numbers are right in line with Dave Winfield's career. His numbers year to year look good but not great, but his career numbers look fantastic.
Or try this one. Say there is a company with a sales team. Maybe they have 32 people in sales or 100 people, take your pick. Each year, they reward the Top 3 sales people with sales awards. In 15 years, Gore would not have won one of those awards. Lots of other sales guys may have won multiple sales awards, yet Gore did not win any. At the end of his sales career, Gore might get a meritorious service award, but he was never one of their top producers. In NFL terms, he may have been Top 10 at his position three times in his 15 years in the league. IMO, that puts him in a very good for a very long time category. Clearly what I think won't make a bit of difference, as he's going into the HOF.
But in my mind, he belongs in the Hall of Very Good. There are a lot of strong players in the Hall of Very Good. That's where I would slot him (and many other actual HOFers). But in MY Hall of Fame, I have first and second tier guys in the HOF and third tier players in the Hall of Very Good. The actual NFL HOF doesn't make that distinction. Either way, he had a great career (as many have noted, RB's don't play 15 years). Personally, even if he is the nicest guy in the world, I don't think his yearly numbers were elite, and when retelling the story of the league's history, there's not a lot to tell about the teams he's been on. Not his fault, but still not untrue. The way HOF voters tend to vote, they like guys from winning teams . . . especially teams that won multiple times. Again, not Gore's fault, but it is what it is.
Henry Ellard also had a solid 15-year career and was even a two time All Pro. When he retired, he was third all time in receiving yards. Yet he's not in the HOF and has the same problems as Gore (didn't play on great teams in his time with the Rams and Redskins). Although I guess Ellard stands a chance this year with the expanded HOF class of up to 10 veteran players going in this year (NFL Year 100).