Let's try a thought exercise on the mechanics of what may have happened here.
The Witcher 3 is currently being sold on Steam, GOG, and other secondary platforms. From the information we know so far, it appears that, after an unsuccessful round of negotiations, CDPR has decided to not supply keys directly to GMG.
From the perspective of the business, we have the following scenarios:
- A sale from Steam, through which CDPR obtains approximately 70% of the price of sale
- A sale from GOG, through which CDPR obtains an undisclosed percentage of the price of sale. Presumably, being vertically integrated with CDPR, that amount is above 70%.
- A sale from other retailers and key sellers, who buy keys wholesales from CDPR at an undisclosed price.
Conditional on selling outside of Steam, it stands to reason that customers are more likely to be price-sensitive after leaving the predominant PC platform. GMG, in turn, has built a reputation for having the lowest prices in the market.
This, I assume, comes through by employing a razor-thin margins approach. They do not sell console games, do not ship anything, and work their business by cutting their own margins to the bone for most major new releases. Assuming very thin margins, then, we may expect their share of profits to sit anywhere between 25 and 33%, depending on the game -- roughly in line with Steam.
The margins from selling on Steam are fixed at 70% after Valve's cut, but is a function of the price, which encourages CDPR to avoid a price war. Furthermore, a faster speed of price decreases also decreases the competitiveness of the platform that's likely to have the largest margin for them -- GOG.
By attempting to block out GMG, then, CDPR can cut off the head of price competition in the PC gaming market: Prices are set at $53.99, with additional price differentiation for owners of the previous games, and in turn also obtains a proportionally larger share of the customer base on their own platform with higher margins.
In short: GMG's pricing practices can lead to lower prices from everyone else in general. But this particularly affects CDPR, since the competition with their platform directly affects the margin that they get by selling on GOG. Selling GOG keys on other retailers should be somewhat in line with selling a key in Steam -- the retailers get a cut, or Valve gets a cut. What GMG did was give nearly all of the "cut" back to the consumer. But in this case, moreso than with developers and publishers that do NOT hold platforms, their margins are more impacted than they otherwise would have been.
Restricting GMG from selling keys was a shrewd business move. But GMG didn't choose to just not sell the game, and so here we are.
Strictly based on the information we know -- It's ridiculous to excuse either CDPR's business decision to not sell keys to GMG, or GMG's decision to resort to another seller. There are two separate, if connected, issues to be analyzed: The negotiation with GMG and their subsequent move. One doesn't excuse the other, and we need more information to actually analyze either without making quite a few assumptions like I did above.