Official rights for GunZ cost more than 100k USD and that's only if MAIET are willing to negotiate a deal with a private server. So good luck with that
Getting a 14 year old game on Steam is one thing. Advertise it and get people to join it is another and believe is gonna cost tons of money. GunZ in general isn't worth anyones money at this point. It has countless problems.
We all loved this game but we are here only for the Nostalgia. Eventually this game will die and it's a shame. One of the most Skill-based games I've ever played.