Old GOG - We get you the games you want
New GOG - Buy what we tell you to buy

Well, they did get Diablo and Warcraft 2 this year so I'm cutting them a bit of slack. But yes, GOG's curation seems weird at times. More pixel junk and less old games than I'd prefer.

Just had a really weird bug where I went from typically doing 2 damage on average to about 99,000 against a boss that had about 230hp. Anyone have any idea what happened here?

https://youtu.be/O3FISV373Xg

Unless it's something repeatable and game-breaking, you either misunderstood a game mechanics, such as powering a slow charging skill that does tons of damage, or lethal damage skill. Or it is indeed a game bug.

I see the boss has over 300HP and you deal only 2 points of damage. That sounds like broken game-design if intentional, so another possibility might be that the boss has a shield and once you whittle it down, you deal massive damage.

But, without playing the game I can only make suppositions.