Why does gog say we can play games offline but the installers or downloads do not always work. Nothing is ever done to fix the problems nor is service provided. I do not play online. I am a single player. That is the way I like it. I have purchased hundreds of games on gog over the years but it seems they don't want me to play the games at home alone and if not on line.

The most recent example is this notice after downloading offline content through their stupid gog galaxy app.

"The base game was not detected. Please install the GOG version."

I have a new pc that I built to play games. I have been trying to download some of my games from gog to play on this new pc but gog provides that impediment for what reason and they do not respond to inquiry.

Any help would be greatly appreciated.

The installer isn't broken. And yes, you can totally install+play your games offline.
The problem is, in this case, you tried to install a DLC (=additional content) before installing the base game.
You have to install everything in order for this to work. Either there is another setup.exe or your offline download isn't complete yet.

Invalid user error.

I would presume your issue lies there.

Downloading the Offline content via Galaxy does not activate it in Galaxy.

Galaxy works in two ways, which can be a bit tricky to understand.

(1) Install during downloading with Galaxy.

(2) Download Offline Installers, which you then manually install. After which you can point Galaxy at the game's install folder, and so use Galaxy with the game.

If you download extra Offline content, like DLCs, then you will need to manually update your game, unless you've already pointed Galaxy to your game folder, which it has then recognized, and it can then download & install the DLC for you, but not the Offline version.

Galaxy is not needed. Use a browser and ignore the big blue tab in the game section. Under it in grey is the offline files. Get all of thwm and that is the offline installer.