I just have to say that, despite usually not really commenting on anything, the comments here that are displeased with the lack of DLC and the soundtrack made it seem to me like you get those for free on Steam, which made me think thrice before i eventually decided to buy it on GOG as well, so i just wanted to clarify a few things (the ones i'm 100% sure on).

Just as a more comprehensive decision making tool, rather than just "how dare they not give me more free stuff" comments.

AT THE MOMENT!, the GOG version is 75% off which makes it 3.09 eur, the base price is 12.19 euros, the Steam version is 12.89 euros without any discount which makes it 70 cents more expensive. Furthermore, the steam version comes with the option of purchasable OST (3.99 eur) and High Crimes DLC (4.99 eur).
You can also buy a bundle which includes the base game, the DLC and the OST on Steam for 5% off, for 20.78 euros at the moment.
The Dark Castle DLC comes free with both the Steam and GOG versions.

I can't say if the version on GOG is as up to date as on Steam, but i can say that the custum levels seem to be linked to the Steam Workshop, since back when i played it, it had the Steam workshop logo in the game.
However, i played the game way back, so i can't confirm if it is in fact still linked with the workshop.

I bought this game on Steam back when it came out simply because i couldn't wait for the GOG release, i just wanted to play it so much.
Back then we didn't have the two content DLCs but i did buy the OST (sold separately) because i've enjoyed it so much.

The game itself was just the perfect length for me to not get too tired of it and was pretty much bug free on my end.

But when i finished the game i said to myself, "man i will definitely wanna play this again, but i want the DRM free GOG version, since i don't want to use the clunky system hog that is the Steam client".
Nothing against those that prefer steam, i just don't like it myself, so i avoid it whenever possible, that's why i aim to purchase every game that i own there on GOG as well (eventually, when i can afford it) since besides my displeasure with the Steam client, GOG offers the install files for backup and Steam does not.

I've read the displeasure of many with this publisher (not GOG or the developer themselves), and i can't really comment on that since i dont own any of their other published games.

But things may change, don't be so certain that because they have lacked the support so far that they will continue to do so forever.

Just make your own decision based on fact, not misinformation, as for myself, i think that 3.09 eur for this game is a bargain and i have bought both the Steam (full price) and the GOG versions.

Party hard is a simple game with a nice retro artstyle and enjoyable music. Although i am curious about the DLC, i would be fine if they never released any additional content on top of just the base game, and i would still have made the purchase on GOG i made today.

I apologise for the rant.

Thank you for reading, and i hope i was at least a bit helpful in you making the decision to either buy or pass on the GOG version of Party Hard :)