Unlike MSVC and Turbo C++, GCC has much less focus on library functions that are not part of the standard C and C++ libraries.
So you should look first to documentation of the standard C and C++ libraries, such as at
GNU also has many extra libraries (multi precision math etc.) that are not part of the C and C++ standards but even though they may be developed and tested with and for use with GCC, they are not documented as part of GCC (because they aren't part of GCC). Look to the documentation for the specific extra libraries you want to use.