This might be a dumb question, but I've always wondered, also to be clear I don't use Galaxy so I'm downloading offline installers.

We get patches to download updates to main games but not for DLC. Most times I haven't played the game in a while and have to download the full installer anyway, but my question is will the patch be enough or do I have to re-download all DLC as well?

This patch (for Grim Dawn) will bring me from 1.1.42 to 1.1.50, the DLC download links also say 1.1.50. Does the patch work for them, or is it required to download all DLC, and if so why no patch for them?

Think you answered your own question. If you haven't played for a long while then you may need to download all the DLCs again. If you're just going from 1.1.4.2 to 1.1.5.0 then the patch will be enough.

So it does patch the DLC(s) as well? That's what I really wasn't sure of.