An entity which raises the ire of Facebook, Google (Web search, YouTube, and/or the Android marketplace), Apple (iOS marketplace), Amazon (AWS, publishing, retail), Cloudflare, other major hosting and service providers, etc., will have a hard time or high costs reaching an extensive audience.

Gatekeeping isn't specific to a single role or function, it's a property which emerges through cost structures, network effects, access to capital or talent, regulatory and political sway or vulnerability, and numerous other factors.

In an age of print, presses, paper, and distribution were limiting factors. Where these were controlled, fixed costs of production (writers, illustrators, editors) were amortised over the large number of copies which could be produced of a single issue or publication, and advertising attracted on that basis. Today, technical expertise in building infrastructure and support systems similarly is difficult to attain, but amortises technical staff and paid content (if any) across not and audience of merely thousands or millions, but of billions in the case of Google and Facebook.

It's possible for the equivalent of a zine to be produced and served to a much smaller audience (though one which would have made publishers of the 1860s, or 1960s, salivate), and at comparatively high per-member cost, though the absolute costs could still be low in most cases.

(Static sites and intelligent caching strategies can still make very-large-scale services on a shoestring budget viable.)

Discovery, promotion, engagement, and visitor insights / metrics are other areas in which today's media monopolies also acquire gatekeeper status.