Sorry, a big rant here (not big in words count but more in exasperation).

We all have examples of devs and publishers reluctant to support at the same level gog and their customers.
So why should prices be equal? I don't think the "just wait for a sale" idiom is the right answer, because the other store can have sales too with that same prices.

Gog should rightfully ask publishers and devs for an inferior price if they want to give an inferior product, or even give them a inferior share to punish them.
Delisting could be an answer especially when they can have a very minor visibility at the other store.

The things missing I refer to, are most multiplayer and modding, especially when these are DRM'ed.
Would achievements be included in that idea, I don't know, I don't think it's the main priority against the other store.

The status quo can't stand for ever.

Well, we vote with our wallet, so as long as people keep buying "inferior" games from known "inferior" devs, status quo ain't gonna change today... not tomorrow either.

A new bloody revolution is your only hope!

People aren't aware of these problems so can't make an informed choice and the market sinks to the lowest common denominator unless you have outside intervention.