If GOG Galaxy 2 could somehow confirm the games you have in other stores, then if the game is available at the GOG store it could download the GOG version of the game instead of downloading the game from the store you bought it from.
This way you wont even have to open the launcher/store you bought the game from. Like this, the hassle is decreased even more.

To be honest, If the launcher has to open another launcher to open a game it's kinda pointless to me. Better to just straight up open the right launcher. Although it's messy if you use more than 1 store, it's still faster this way.
I do appreciate centralizing the libraries and Galaxy 2.0 current design, though in the end it would just be another launcher I'll need to have installed.

I'm not sure how much of this is feasible though, but hopefully it's not that complex or it inspires new similar, more feasible ideas!

Doubt they can do that, the pubblishers will rather have you buy the game twice

There would be a problem with the rights to own a GOG copy, as GOG is DRM-free and other stores require software to run. There is a service that is ran by GOG called GOG Connect that connects to Steam and will add applicable, approved, games to GOG from a changing list. Not sure if it's still being updated though.

I thought at this point people would wise up to the idea of GOG giving away free copies because you have a game on a different storefront. It will NEVER happen.

I don't want this to happen - if you like a game as much, then wait for GOG Connect or buy a second/third/47th copy of it.
If not, then just use GOG Galaxy 2.0 and launch it from whatever client you use now.

Don't be cheap

It would be an interesting feature to help wean folks off other stores if GOG Connect’s functionality was part of GOG Galaxy at least. Something like a button/badge for supported titles.

Just open your account, choose game and click on "Download offline backup game installers", and then click on the files on the list to download them. If a game contains multiple "part" files, you need to download all parts before you can start the installation.

GoG already have (or had) something like this... Its called Connect (I cant post links, but a Google search for GoG Connect should bring it up).

Its not for EVERY game (actually not many games at all), but GoG do talk to developers/publishers to try and make it happen.

Whenever an agreement is reached the games added to connect and you can add the game to you GOG library.

It hasn't been very active though so it would seem not many publishers are into the whole idea.

All they need to do is integrate it into Galaxy 2.0.