how to use __int128 in gcc.

What do you mean? It works just like any other object type.

> how to use __int128 in gcc.

Use the GNU compiler default: ie. no standard C++ conformance

__int128 is not supported under standard C++ http://coliru.stacked-crooked.com/a/e19c0598ee818039

Even with the home-grown dialect, there is no library support for __int128
http://coliru.stacked-crooked.com/a/b5fd627d3a7376f2

Consider using boost::multiprecision::int128_t instead (portable, standard C++)
http://www.boost.org/doc/libs/1_60_0/libs/multiprecision/doc/html/boost_multiprecision/tut/ints/cpp_int.html
http://coliru.stacked-crooked.com/a/92c1c439b7b6e5a3

There is some library support: std::numeric_limits are defined.