Please note that C is quite a few years older than the first GC languages. LISP1.5, ALGOL-68 and APL all had garbage collectors before C even existed.

Not to say that it's not worth it to learn manual memory management as well, but it is important not to think that GCs are a fancy modern tool, and that greybeards would never touch one. There were greybeards using punch cards and programming in a GC language, with output printed out on paper.