You have to look at both parts of what he said, not just the first part:
wants him as the power 4 beside a shooting center.
People are ignoring the "beside a shooting center" part. We of course do not have a shooting center (well I guess Kornet but he is pretty much 3rd string) so the whole statement is a bit moot but if we did have a truly offensive minded center, I think Williams could play alongside of a player like that as the second big. Say someone like Sabonis or even Embiid.
That said, I think the more natural fit is Williams as the Center beside a "shooting power forward" but in theory, it could be either way.
Right now, Williams seems to be used the most beside GWilliams or Ojeleye (shooting PFs who aren't big enough and don't shoot well enough to really qualify) or more recently (since losing Theis) as the only big. Williams is definitely more center than PF to me but in the specific case that Gorman references, I think it would work.
But what are you gaining?
What advantage to you gain individually for Rob Williams or collectively with Rob Williams as a PF vs at C.
In other words, what is the point of it.
As far as I see, 95% of the time it is a losing proposition for the Celtics. Why?
Because individually Rob Williams spends more time defensively doing things he is not so good at (defending on the perimeter) and less time defensively doing things he is good at (shot-blocking). The truth is that a more natural forward will add more to the team defensively than Rob Williams does because that forward will be able to defend in space better than Rob Williams can so team D will be superior.
So do you gain offensively? Rob Williams has little scoring ability so there is no change in his individual output. So is there a benefit to team offense? By going bigger and slower? It would take a hugely skilled (and likely mobile) center to make that work and even then it would still work better with a more skilled forward who can add to the offense more than Rob Williams can. So team offense will be superior with a natural forward.
Rob Williams is in his perfect position as a center for today's NBA.