Does anyone know if the Battle Rock Nemesis content is included or not in the GOG version of Overlord 2? It would be very helpful if this can be answered.

No. That DLC was only ever released on consoles and never was in the PC version. GOG is selling the PC version.

There's been some attempts over the years with reverse-engineering it, here:
https://steamcommunity.com/app/12810/discussions/0/2666627464749404317/

In May, someone managed to decompress the files from console version, but still nothing tangible.