Gog is great for preserving games that were made and published before 2009 - basically games that don't have (or neccessarily NEED) a modding scene, but unless you know EXACTLY what you're doing, you wouldn't be modding most games you buy on Gog anyway. Old games are great on Gog. New games (post-2010), you get on Steam, because Mod Workshops are a huge part of certain games. Going Medieval is indeed one of those games. Going Medieval does have an amazing mod scene, but unless you buy it on Steam, you're missing out on a giant chunk of community made stuff that you won't have access to in the Gog release. Don't make my mistake. Game's great, but buy ON STEAM!
Zapper is indeed a videogame - one of the videogames of all time. I remember pouring hours into this game and beating it. I played that game so much as a 7-year-old that I remember each levels' opening stages almost 19 years (as of writing this review) later. I also remember being frustrated with this game and I remember that it was an ok game that once I beat as a kid, I was satisfied to put down and never play again. Now I feel as if God himself is nudging me in the shoulder to try it again, but I really don't feel like it. To tell the truth, I'm actually shocked anybody other than me remembers this game even exists because I remember almost nobody knowing about, let alone playing the game, so seeing it here on GoG is kinda reality-warpy but in a way I'm ok with.