Your main focus should be old gunz players since "new" gunz players are either non existant or a species yet to be discovered.
The best ways to reach out to these old players wouldn't be websites that relate to gunz in anyway since if they've lost their interest in GunZ you won't be finding them there either. Decent ways to promote an old game that is fading into oblivion would be making use of the registered accounts and sent mass emails inviting them back, I see that FG has 15k likes on Facebook so promoting there is a good idea as well.
Other than that make use of huge forums/message boards ***** etc and grab some attention there.
Goodluck finding old/new players! being more on target what you want to achieve should help so therefor I do hope that you find this post usefull.
