You have to consider where the two get their shots. Gasol not only produced at a very high level in post-ups and isos (0.96 PPP and 0.86 PPP, respectively), he got a lot of his offense out of these situations (43.7% of his possessions that ended in shots, shooting fouls or turnovers). Keep in mind that these are, on average, low PPP possessions (along with pick and roll ball handler...together averaging about 0.8 PPP), although the creation of other halfcourt shot types (~1 PPP) are dependent on plays initiated through isos, pick and rolls and post-ups (between 38 and 49% of team's halfcourt terminal possessions came out of isos, pick and roll ball handlers and post ups). Gasol was also very effective in pick and rolls (1.19 PPP, 20% of his possessions). Compound his individual brilliance scoring on-ball with his passing, and you have an elite offensive player.
Favors got much more of his offense off-ball, in transition and via offensive rebounds (Gasol's O-boards are presumably nerfed by ZBo...Memphis was second in the league in offensive rebounding percentage). Those opportunities are converted by a lot of players at a high rate, come out of lower leverage situations, and thus add far less value to overall team offense.
Despite these advantages, Favors' 0.9 PPP total falls far short of Gasol's 0.97 PPP (without any consideration of the value Gasol adds to others through the type of possessions he uses and the assists he racks up).