I created a barbarian PC and he has over 400 hit points - I vaguely remember this being an issue when the game was first released.

I suspect that the GOG version doesn't include the final patch (or perhaps the final patch was unofficial?) or maybe it's because I'm running in multiplayer so I can choose all 6 members of my party?

It is patched with the latest official(26498) patch. There is a 26499 but it's only beta, and fixes only an issue with save/load times. I think it may also break mod compatibility.

Edit:
Or did you mean BG1? Not sure if you could create a barbarian there, I played only with EasyTutu in my last playthroughs. BG1 is also patched to the latest version as far as I remember.

If you have the BG2 tweaks mod option 'max hp for PC' it will glitch the barbarians HP.

I have the same glitch with the 'NWN-style lvl-up' (another setting of the same tweak from BG2 Tweaks), it even happens with a fully patched game.

My solution (maybe I should say workaround) is to use Shadowkeeper to set the Barbarian's hp back to normal after every level-up.

BTW: isn't it the policy of GOG to bring out fully patched versions of the games they sell? So patching is only needed on the disc-versions, not on the GOG-version?