It's hard for me to justify a purchase without all the DLC on Steam available on GOG. I see no mention of the following DLC with the GOG release:

Two Point Hospital: SEGA 60th Items

Two Point Hospital: Free Zombie Costume

GOG and Sega shouldn't be discouraging purchases from their potential customers by treating them like second class citizens!

There is more DLCs. Golden Toilet as Sega 60th site newsletter registration.
http://steamdb.info/app/535930/linked/
it didnt even listed here
http://steamdb.info/app/1185851
or it is part of Hospital Pass...

Zombie according to PCGW and store pages descriptions for fancy costumes DLC is included to MS Store EA Origin and now GOG versions as part of another DLC;
Sega 60th is Steam only looks like.

Anyway. Both Steam and GOG - and so any else PC(-W\M\L) versions, all DLC is jus unlocks. They all already in the game files.
It not mean they all can be accessible for offline GOG build. But *something* can be apllied to GOG version just like for Steam one. But ofc it be not 100% clean game.
But first its b better if someone can investigate content for all items of all 15 ingame DLCs.

Thanks for letting me know. The whole DLC situation for their game could've surely been handled in a more consumer-friendly manner.

I've decided to purchase the game and DLC on GOG despite my disappointment.