The statistics indicate that Gordon has been roughly half as productive in far less than half the time/games played.
It seems to reinforce the argument that Gordon has more ability than Thomas (although not by a big margin, Thomas himself is a very gifted WR). This isn't/shouldn't be a controversial opinion; Gordon has the ability to be one of the best receivers in the history of the league, his issue has been staying on the field.
All that said, Thomas has been much more consistent/reliable throughout his career insofar as staying on the field.
DT has 9,154 career yards and 62 TD's (127 games) vs Gordon's (52 games) 3,826 yards and 19 TD'S.
DT averages 72 yards/0.488 TD per game
JG averages 73 yards/036 TD per game
JG's averages are vastly inflated by his 2013 season and DT's averages are deflated by his first two seasons when he started only 7 of the 21 games he played in.
Without 2013, Gordon averages 57 yards/0.3 TD per game
Without 2010/11, DT averages 76.5 yards/0.5 TD per game
I don't like discussing the hypothetical, i don't mind that you think that Gordon could have been an all time great, maybe he could've, but we'll never know.
What i do mind is saying that Gordon was a Top tier receiver while DT wasn't which is simply false:
2012-2015
DT 5787 yards/ 42 TD's
Antonio Brown 5818 yards/36 TD's
Julio 5242 yards/26 TD's (missed 11 games in 2013 due to injury)
Now whether he can be even 60% of what he was is an entirely different matter.