NixOS and Gentoo: systems to build yourself

In short: NixOS describes the whole system in a configuration file. Gentoo builds all programs from source code on your own computer. Both are for advanced users.

In more detail: What they share: maximum control through configuration.

In detail

• NixOS: declarative. configuration.nix states which packages, services and users exist. nixos-rebuild switch rebuilds the system. Every generation stays as a boot entry (rollback possible). Packages in /nix/store separated by hash, so no dependency conflicts. Flakes for reproducibility.
• Nix outside NixOS: also usable as a package manager on other Linux systems and macOS, nix-shell and nix develop create development environments.
• Gentoo: Portage package system with emerge, USE flags decide which features are compiled into a program. Long compile times but tailor-made. Alternative: binary packages.
• Learning value: Gentoo teaches how a Linux system is put together, NixOS how systems can be described reproducibly.
• Disadvantage: steep learning curve, lots of documentation to read.

See also: Immutable Linux systems (immutable distros), Building software from source (make, CMake)