While there's optimism and enthusiasm for helping ZS shown in what you wrote, welchnome, I'm sorry to say that simply creating and signal-boosting ZS content online won't be enough to help boost population numbers. Nor will simply banding communities together under a roof for ZS help it out much. And this has to do with what ZS actually is.
GMod's an old game, and a lot of people have moved on from it. The Zombie Survival game mode is also old, and as some have said before, ZS is a niche game mode. Very few people are interested in it, and that's because there are far better video games than ZS to play and get addicted to. Those who have been on our ZS server for a long time have come on-and-off ZS for how addictive it is, but that's because it's addictive to them. They found it addicting. That doesn't happen with everyone. Someone could try it out and have fun with it in the first 2 hours, but after that, they might grow bored after discovering how boringly repetitive it is, then leave.
Content, in the form of new things to do in a game, are what drive people in. Marketing a game is just as important as its development, but if there's no new content to advertise, then there's nothing to market. Even if some popular content creators are brought in to help revive ZS by creating content for it, what about ZS could be interesting enough to attract an audience? And can there be different, potentially entertaining ZS content that can be made on a weekly basis with what ZS currently has?
And keep in-mind that not everyone knows ZS even exists, and I can see where you're driving at with content creation: the goal is to make it more well-known to attract people to it. The greatest limitation to that is what ZS actually is. You can advertise/market it all you want, but if people don't take interest in it, you throw away a lot of money and time. Unless more gameplay content is added to ZS, and its overall game design and stability are improved, you won't get far with just marketing on its own. You need development as well.