I purchased the standard version but would really like to upgrade to the deluxe without having to buy the entire thing again.

Well, the owners of the deluxe edition are still waiting for the extra goodies... I got the Delux and so far no artbook or soundtrack

Wow that sucks. I'm almost regretting purchasing on GoG. The no DRM is nice but it seems to be entirely ignored by Croteam here. The devs are very active on the steam forums but nothing here.

Steam users don't have the Deluxe content yet either (or haven't had since I last checked). I read a statement that they are finalizing the soundtrack, including some extra tracks and then they will put it out. I am slightly annoyed too that it wasn't ready for release but if it comes soon-ish, it's whatever.

And about dev activity, just compare the forum traffic. There is really not much happening here compared to the Steam forum. Just be glad you could get this DRM-free at all :P

Artbook and Soundtrack are available for download from your account page. They're not available via Galaxy yet.

It's not on my account page yet...

Looks like a bug on GOGs side. Try doing an account refresh by clicking on the following link, wait a minute or two and then reload your library.

https://www.gog.com/account/refresh

Soundtrack and artwork are there for me (see attached screenshot).

I can confirm that worked. Thanks :)

Well, it's now available on the store, though not listed as DLC on the standard game's page. Not sure why it was not there from the beginning but alright.

https://www.gog.com/game/serious_sam_4_deluxe_edition_upgrade