A few days ago, I tried to play a private online game with a friend, and was unable to see a single game in the server browser. I verified the Galaxy client and the game were both fully up-to-date, and that there were no firewall issues. Either way, I still couldn't see anything in the server browser, and gave up.

Today, I am unable to even access the server browser, as the online option is greyed out. LAN seems to open fine. I'd just do Hamachi, but my friend's Hamachi install seems to be completely messed up. I've restarted the game and computer multiple times. I couldn't find any sort of log to look through.

Anyone experiencing anything similar or have any advice? I emailed GOG about the empty server browser issue, but was only told to update my client and unblock any firewalls. I emailed Gearbox too but am still waiting for a response...

This happened to me too, when I opened up the launcher and hit Multiplayer. The LAN browser opens fine, but Online is greyed out and unable to open. I did notice after closing the game that a Windows notification had popped up warning me about Windows Firewall blocking parts of the program, so I clicked on the Allow Access button. I tried booting up the game again, but the online multiplayer is still unavailable.

This is for HW remastered. I own the originals for both and have used the Direct IP connection before, but was hoping to play remastered.
Didn't know Gearbox didn't manage any of the servers, though. Thought they went to the same set of servers but allocated to different pools. I checked my firewall properties too, but yeah, glad I'm not the only one.
I also should mention I've reinstalled the game, which didn't help.
I'll check again after work. I was hoping it might be a temporary server issue (but why did I get no sort of indication or message besides the option being unclickable??)

Nope, the servers are managed by the distributor's own networks (ie Steam and GOG.com). If Gearbox really managed the multiplayer servers, then it wouldn't matter if you had either version, as multiplayer would be unified, which isn't the case, as people with the GOG.com version can't play against those with the Steam version and vice versa. ;)

As other users are having problems, you might want to try contacting Gearbox, but don't expect them to be able to help much, as most of their efforts are focused on the Steam multiplayer. GOG.com Staff should be able to assist though, assuming they care enough to post in this thread.

I'll definitely be contacting them again and point them to this thread. I'll update if I find/hear anything or it starts working again.
Edit: Checked again just now after work.. still greyed out.

Likewise, it's greyed out for me.

And in a new twist today, all of my saved games from the HW2 Remastered single player campaign have...disappeared. Any excitement I had when first Steam and now GOG released this "Remaster" has quickly faded.

Just checked, the Multiplayer online option is available again. Hopefully will have the chance to see if it is stable tonight.

Edit: False alarm... it's greyed out again. No clue what happened there.