A see a lot of posters saying we run a gimmick offense and that it's been "figured out".

Maybe it's just my perspective that "gimmick" suggests lack of legitimacy, but running the play action to set up the pass seems like a pretty solid idea.
Where we fail is that we really don't have a running game to give the play action credibility. The lack of running game falls on a weak OL and non-threatening RB's. I don't see it as coincidence that our offensive woes mirrored Todd Gurley's decline.

If you can stop the run with your LB's, then there is nothing to freeze the DL and they can proceed with pressuring Goff into poor play. So I guess it has been figured out, as such.

Has McVay "dumbed down" his offense. Of course, because due to average players and poor execution most of his playbook doesn't work, not necessarily because of Goff's struggles. (and he is struggling, don't get me wrong) But the offense, at it's core, is still quite valid.