So I have found many games in the GoG search results I would like to buy from nostalgia. For instance "Bubble Bobble" or "Zool 2", but when I click on the search results, there is no buy button?

So it seems that the galaxy search icon at the top of the side panel returns results for almost every game in existence. Unfortunately GOG only sells a tiny fraction of the listed games.

GOG galaxy.was originally designed as the client to end all clients, listing all games across different store launchers in a single place. That never eventuated and linking to other stores (even ones with official integration) is strictly forbidden.

If you want to search for games that are actually sold by GOG, click on All Games and use the search box shown on that page.

Or you could just look on the normal GOG site, without using the galaxy client.

Heresy!!!

Galaxy shows results based in a database, doesn't matter if it is available in GOG or not, but obviously the buy button will only be available for those titles if they are purchasable in GOG.

This feature is basically useful for adding games to your Galaxy Library externally, this is, for example, physical versions you own.

Obviously any of the the proper Galaxy features like autoupdates or cloudsaves won't be available for these games, but will make those games visible in your library. They will be shown under "others"