Gentoo, while source-based and having an interesting approach with USE flags, does not come with NixOS' strengths.

I'd even say that Gentoo's packaging might be better in some aspects than that of nixpkgs, which does feature options that you can change via overrides but generally isn't as modular as Gentoo's system. But the mistake a lot of people – and I'd say you as well – make is that they look at the wrong parts for comparison, and don't understand what makes NixOS so powerful. It's not the sheer amount of packages or how they're built, but rather the module system, the declarative nature and the option for rollbacks at the "package manager" level. Yes, these features come with increased complexity. However, I recommend not to look into what people have published in GitHub as their configurations, as these are rather general and as such more complicated than one needs for casual use.