What are the GCC default include directories?

In order to figure out the default paths used by gcc/g++, as well as their priorities, you need to examine the output of the following commands:

1. For C:

    gcc -xc -E -v -

2. For C++:

    gcc -xc++ -E -v -

The credit goes to Qt Creator team.

There is a command with a shorter output, which allows to automatically cut the include pathes from lines, starting with a single space:

$ echo | gcc -Wp,-v -x c++ - -fsyntax-only
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.8.2/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2
 /usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2/x86_64-redhat-linux
 /usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2/backward
 /usr/lib/gcc/x86_64-redhat-linux/4.8.2/include
 /usr/local/include
 /usr/include
End of search list.

The credit goes to the libc++ front-page.

To summarise the other answers:

For C++:

c++ -xc++ /dev/null -E -Wp,-v 2>&1 | sed -n 's,^ ,,p'

For C:

cc -xc /dev/null -E -Wp,-v 2>&1 | sed -n 's,^ ,,p'