Is there a way to deactivate the Ridiculously Bloody Blood Pack for Dawn of War 2 Retribution?
Perhaps a file that could be deleted or edited?

I use Linux and therefore cannot use Galaxy. (In case there is some setting there)
I installed it using the offline backup installer.

Any help would be appreciated.

Went to Gog support with this and they said there was no way to disable it. You can try contacting support, maybe if enough people complain Gog might actually fix the issue - probably not though.

I don't own the game, so I can not try anything myself.

From what I understand, the "Anniversary Edition" on Steam and GOG contains all (?) available DLCs already?

And on Steam at least, the "Blood Pack" DLC seems to be quite easy to disable (quote):

WARNING!
This content is EXTREMELY bloody which can affect battle visibility when in combat.
This option can be turned off from the Steam library
-> Dawn of War II - Retribution
-> Properties
-> Ridiculous Bloody Blood Pack
-> tick box (checked = on, unchecked = off)

I take it, the (non-Galaxy) GOG version doesn't have an in-game option in the menus to enable/disable the included DLC(s) either completely or to disable/reduce the blood effects at least?
That sucks, of course.

Have you looked into the .ini file?
Maybe there's an entry for the DLC (or the blood), which you can set from "true" to "false" or from "1" to "0", etc?

I see. I have also contacted Support and am waiting to hear back from them. Hopefully they will try to fix it if they see that it bothers enough people. I only see 'effects density' (this can also be changed in the in-game options), but nothing pertaining to the DLC. Setting 'effects density' to low seems to decrease the number of the blood effects, (while also decreasing all other effects). The blood still explodes in massive pools though.