For a Go programmer, it can be very helpful to already know C because then you can use the cgo package to fall back to C when you need more direct connection with the operating system. But then you may be writing non-portable code, so it’s best to avoid cgo if possible.
If you learn C first and then learn Go, you will find Go much easier, as Johan said. But you will still have to learn things that are in Go and not in C, and you will need to adjust your thinking to almost always prefer to use slices rather than arrays, for example.
I learned C in the early 1980s, and as a result, learning Go was easy for me. But if your objective is to learn Go, then I have to say that making yourself learn C first really isn’t worth it.
BTW, I don’t agree that Go is only for people who are “extremely proficient” with C/C++/Java or the like. There are people on the forum who are learning it as a first programming language, or with backgrounds in just Python or some other interpreted language.
I think the thing that’s missing is a good comprehensive book to teach Go starting at a beginning to intermediate level. There are few books (and online resources) for beginners, and they usually don’t cover the full language. And there are books that start at an intermediate to advanced level that go into detail. So it’s easy to get the impression that Go is for advanced programmers only.
Really, I’ve found Go a lot simpler and easier for beginners than C was for me. Maybe that is all I needed to say!