@w96k I'm not a fan of GNU C, but I have no issue with people using it; I use a *different* nonstandard dialect (Ken C).
What I really dislike is Stallman perpetuating the idea that language matters. If it was a reference manual for GNU C, that would be alright, but the advice to use it to learn C is terrible.
If you know basic programming concepts but not C, you do NOT need 200 pages to learn C. You need like 20, of which 10 are about the lang and 10 are pitfalls like the promotion rules.