Fallout: New California — one of the larger, long-running mods for Fallout: New Vegas — has appeared on GOG, and yes, it installs with a single click. It landed there through the Game Preservation initiative, an effort to keep notable projects from disappearing (and not just a neat press line).
The mod transports you to another patch of the wastes: new locations, a fresh plot, full voice acting, a reactive radio system, and eight companions — chaos guaranteed. You start off as someone from Vault 18 and will meet factions like the NCR and the Enclave, plus raiders and super mutants (e.g., scripted encounters that affect later beats).
The project’s first part was released in 2013; the last piece arrived in 2018. Now everything’s been collected into one package and made available on GOG for straightforward installation.