GCC optimization and speedup capabilities

GCC is capable of performing a wide range of automatic optimizations=20
to improve code execution speed, and it does include features for=20
automatic parallelization of loops.

Although I can add explicit code to use threads and parallel processes,
is the gcc and the OS capable of using multiple threads and available=20
cores even when not explicly coded by the developer.

GCC can add optimisations for a program automatically.  Is it capable
of taking advantage of multiple threads and available cores, without
the developer having to code them on a single machine?