As you probably know, WordPress is open source software and licensed under the GPL, theme and plugin developers who create derivative works (i.e software that relies on WordPress to function) are encouraged to adopt the same licensing structure as WordPress, and many do.
This grants users certain rights regarding modification, use and distribution of source code.
The GPL allows users to use, share, modify and redistribute software as they please. This enables us to redistribute software developed by authors such as WooThemes, Gravity Forms, WPML & more for a much lower price than charged by the original authors.
Additionally, every plugin author references and acknowledges that their software is licensed under the GPL within their terms and conditions. Theme Canal is well within the bounds of the GPL license which allows anyone to sell copies of a GPL program for funds (see here).
We are able to offer low prices because we don’t provide support or automatic updates. We do provide updates as released, but you have to apply them manually. So it means that if you require an account at gravityforms.com, and a license key for automatic updates, you should purchase directly from the plugin author.
If you need support or automatic updates, you should opt for the plugin author.