GCC compiler does not search sub-directories when looking for header files

I can't make gcc compiler recognize complex paths in "includes".

Here's my toy "main.cpp" file (note the sub-directory in the include statement):

#include "sub/testlib.h"int main(){    testlib(6);    return 0;}

Here's the path to the file "testlib.h" from the folder "main.cpp" lives in: ../lib/sub/testlib.h.

I'm specifying the include directory while compiling:

gcc -c -iquote../lib main.cpp

And the compiler yells at me:

main.cpp:1:10: fatal error: sub/testlib.h: No such file or directory    1 | #include "sub/testlib.h"      |          ^~~~~~~~~~~~~~~compilation terminated.

Of course I can make it compile by removing sub-directory from the path. But this is just an experiment I make after having failed to compile a real-world project. I can't freely change the files there.

How do I force gcc to treat sub-directories well in the includes? Is there a flag or some option that I'm missing here?