Welcome to XF2 Addons. All the XF2 addons located in the Resources area are available for download once you upgrade your account to Premium membership. In addition, Premium members will get technical support. Premium membership costs only $35.00 USD a year. Once the year has expired you can continue to use the installed addons on your forum as long as you like.
Guzzle is a PHP HTTP client and is already installed with XenForo.
So what does Guzzle do you? Some add-ons need to be able to do things like get an image from a website, or communicate with a search engine. This is typically done by using a PHP command called cURL. What Guzzle does is it wraps code around the cURL so that a developer can use the Guzzle code instead of the cURL code, this is cleaner and more efficient.