Unfortunately the dlc , same as the unlocks, were all contract mode-only
And contract mode was never included in the gog version sadly, making the gog version various unlocks totally useless too.
The steam version contract mode isn't working anymore either as the servers have been shutdown
while there's no hope for gog version of Absolution, people using the steam version may have a possible alternative according to pcgamingwiki :
Servers for online services were shut down in May 2018, making the Contracts mode unavailable, any Contracts related achievements unobtainable and any single-player unlocks useless.[1][2] To play and restore the Contracts mode join this Discord server (
https://discord.com/invite/5WW3C3G2M3 ) made by LennardF1989: Hitman: Absolution and check out the welcome channel for full instructions. (NOTE: This is not a full replacement of the Hitman Absolution servers, some functionality is missing.)