Ask HN: Who can access GPL'd source code

I was following the thread regarding the fallout from the Zen fork of Zig [0]. A number of comments brought up the licensing and how the situation would have been different if Zig was under GPL rather than MIT license.

This got me wondering. Say I have a GPL project and someone forks it. They then distribute their software to paying customers. Does the GPL only require that those paying customers have access to the source? Is there any requirement that the original author also must have access (assuming they aren't a paying customer of the fork)?

[0] https://news.ycombinator.com/item?id=24481142