If you have installed Google Voice and Video Chat plug-in (or Download and install it from here - Requires Windows XP+ / Mac OS X 10.4+ / Linux), You can make calls to any Phones around the world from Gmail.
If you are making calls to US or Canada (it will be one cent to make calls to US and Canada from other countries) it will be a Free service while other countries you will need to pay a nominal amount. You can see the comparison chart here
Calls to the U.S. and Canada will be free for at least the rest of the year, says Google
This feature will be available for the US based Gmail users
Read more here