The title.

yup, i'm still waiting on the songbringer one before i buy. in fact, in the case of songbringer it's more like an expansive update to the existing game than a separate dlc, so it's definitely vital.

this is the response i got from thiev last week:

https://www.gog.com/forum/general/so_are_we_getting_the_songbringer_update_discount_that_steam_just_got

i know gog does a test phase on their patches but i dont know how long that normally takes. in my experience it's been quicker than a week.

The Regalia devs are talking to their publisher about releasing the DLC on gog, wether you want to believe that or not is for you to decide, I'll give them the benefit of the doubt this time. :)
I'll probably ask them about the DLC in a week or two again, that reminds me I have to ask the Agony devs if they are having any luck regarding a gog release.

According to the changelog, Songbringer should have added it in the last update, even though it doesn't appear as a separate DLC in the store...