PHP's GD extension will be using the version of the GD library that is provided with your operating system. If you're using a modern operating system such as Ubuntu 12.04 or 14.04, then you'll be using GD2.
You can confirm the version of the GD library that PHP is using by checking the output of the PHP function
Alternatively, you can test this from the command line with with the command:
php -i | grep "GD Version"
On a server managed by ServerPilot where multiple versions of PHP are available, the command is slightly different as you need to specify which PHP version in the PHP command:
php5.6-sp -i | grep "GD Version"
The output will look something like this:
GD Version => bundled (2.1.0 compatible)