Green was pretty much past the "highly touted prospect" stage when he was traded, and Gomes was a starter mainly due to injuries. They weren't starter material.
Green had just finished his sophomore year, which was the first time he got nontrivial playing time. Perhaps Celtics fans had come to their senses (I certainly didn't expect him to be much of a player), but he still had value around the league. And even if Bradley is arguably a better player right now than Green would ever be (provided his performance lasts through the season), I don't view Sullinger as Jefferson's equal.
As for why Gomes was a starter -- plenty here view Bass as a prototypical backup PF anyhow, and Bass and Gomes were pretty similar in their primes. They're the same player.
In many ways Gerald Green at that time was less exposed as a bust than Bradley is now. Green's lack of playing time and his sport center highlight dunks from his short minutes kept up the interest in his potential, whereas Bradley has played a ton of minutes and it's pretty clear he is what he is at this point, and undersized SG with terrible ball handling and passing skills who excels only at pressuring the ball.
Green was a bust, Bradley isn't.
Right now, Gerald Green on the Suns, a better team than the celtics and a legit playoff team in the West, looks better than Bradley does, and their average stats look identical:
Bradley: 13.8 points, 4.2 rbs, 1.3 assists, 38% 3pt shooter, 45% FG shooter
Green: 13.2 points, 2.9 rbs, 1.4 assists, 39% 3pt shooter, 44% FG shooter
Anyway, Green was not at all considered a bust when we traded him to the T-wolves, his flaws were hidden enough, unlike Bradley who's flaws are very evident.
are we really comparing bradley and green? ok, let's go to it.
in comparing green and bradley, why include team w/l as an indicator of their individual stats/ability? would you say bradley, based on those stats, is better than green should the celtics have a better w/l record? let's keep oranges and oranges together here.
first in comparing these two players, it might be informative to factor in their ages and nba time. how many years has green been in the nba compared to bradley? (hint:7 to 4) their ages are 23 for bradley and 27 for green. when viewing the bradley/green stats above this makes a difference in evaluating the players performance.
for example, in his 4th year, green averaged 5.2 points and 1.4 rbs. maybe we should compare them year by year to gauge levels of development and performance?
green's shtick/calling card is offense. it is pretty much all he does, and it took him 7 years to reach the level you give above. yet, bradley who is clearly a defense-first player matches roughly green's output on offense after 4 years.
next...what about defense? can we factor that into our comparison as well? if we do, i think bradley pulls ahead easily, while having roughly comparable offensive stats at a younger age than green.
bonus point time!... and just for fun, let's discuss how a 6'2" sg (that's bradley for those of you keeping score at home) rebounds versus a sg/sf who is 6'8" (and yes, that means green.)
drum roll please..... green = 2.9 rbs/game versus bradley's 4.2 rbs/game.