The game features the multiplayer trailer on the storefront. I know GOG Galaxy has restored multiplayer in the past, and as someone who got max rank in the MP, I'd be over the moon if this version finally brought that back.

Also, it's additionally unclear if Deathstroke'sDLC and the Black Mask map pack are included or not?

ParadigmFallen, there is a folder of singleplayer only, the folder of mutiplayer is missing after installation. That is why there is no multiplayer in this version. Eleven achievements for multiplayer are not included into Gog version, that is why you can obtain all the achievements.

Hi ParadigmFallen.

I can confirm that the "Deathstroke Challenge Pack", which I believe was exclusive to retail purchases for a while, is included in the "Batman Arkham Origins" base game and you can choose to play as Deathstroke on the challenge maps.
The "Black Mask Challenge Pack" is included in the Season Pass of BAO's GOG.com version, even though it is not specifically mentioned in the store page and is certainly not sold separately, unlike Steam.
There is no statement from the GOG.com team, regarding the revival of the game's Multiplayer and as megatherion888 posted, the complete absence of a MP folder doesn't hint to any intentions of doing so, even though I would happily welcome it.

Cheers.