The pricing of games and dlcs is based on different aspects. There is the revenue the developer likes to have as well as how much he is willing to reduce this price on events dependent on the age of the game.
Another aspect is the 30% GOG gets from every sail as provision. To boost sales, they may resign on a part of this and reduce the selling price with this even more. Here, probably statistics of how many customers bought the main game, have one or more dlcs already and might be willing to buy another one, and for how much they bought the other dlcs and therefore are probably willing to pay for another (the so called "willingness-to-pay threshold") are usually used to find the best price tag to make a maximum profit from 'amounts of sold units' * 'price' while the game/dlc is still 'in scope'.
As customer behavior on steam and gog might be different, so the price tags are.

Nothing is wrong with those prices. Buy or or don't - doesn't really matter...