It's not the case that the online multiplayer artificially requires GOG Galaxy. The online multiplayer, achievements etc. in many new games are built using GOG Galaxy. Those features also require a consistent user account. Online multiplayer of course also requires you to be online.

For each of those games, the single player part of the game can be played offline and without GOG Galaxy.

Developing a new multiplayer platform for every game separately is a massive undertaking for a game developer, and is mostly a redundant work.

Achievements work, when you're logged in Galaxy. They don't work if you don't use Galaxy. They are not mandatory in any way.