Breja: I think that a game that sells the final boss as a DLC should not be purchased at all, anywhere.
If they say something like "buy the DLC to finish the game" at the end then that isn't cool but it seems like a quite unfair accusation to me. I didn't get to the end (after getting stuck trying to get the gold on a particularly difficult level that was later modified or removed) but it is still one of my favorites and well worth what I paid for it, over double the current price and I still have a number of levels to go. There aren't a lot of games like it and I found this one quite fun. The developer released the DLC a year and a half after the main game and it has dozens of new levels not just one battle (and I'm not sure how much of a "battle" it is, there aren't really battles in the base game but it sounds like there might be in the DLC). I still think two plus years of work is better for DLC but this seems way better than DLC after six months, or at release, or worse during early access.
The game has very little story, it is a speed run game where the entire point is to get a good time on each level. Offline there are three times to aim for that felt nicely balanced to me other than the one that I got stuck on that has been changed. You can save what you did to review to figure out how to improve. The levels are nicely designed in my opinion and swinging around is fun. I can't remember if I had some nausea playing it but it was at least much better than any first person Unreal Engine game I can think of. I enjoy speedrun games but rarely finish them entirely.
So I say get it on GOG since there is a lot of great content in the base game and if you make it to the end and like it enough to insist on playing the DLC elsewhere (maybe send a nice note to the developer first to try to get it here) then it will have been fun enough that handing the developer an extra $2 to rebuy the base game isn't likely to ruin your day.