Not sure if that was something GOG specific, but basically had to download the whole game again.
The official specs are that the patch is 30GB, but requires 130GB free disk space.
GOG has has some strange calculations it performs for free space, particularly for its offline installers. One of the Cyberpunk 2077 updates required 400GB+ free space to install a sub 50GB patch, which was ludicrous and I think under-reported due to most using the Galaxy install process.