The GNU General Public License (GPL) allows you to use and modify Essential within your organization but does not require you to release any of your changes, if you do not wish to. However, if you release the modified version to others outside of your organization then you must make your changes available to them, and attribute the original code to EAS Ltd, under the GNU GPL license. This is known as a reciprocal license, and we have opted for it to protect our product and ourselves. We did not want another company to take the tools of the Essential Project, make a few small changes and then market it as their own product without the benefit of it being open source.

The following is an extract from the GNU GPL FAQs:-

The GPL does not require you to release your modified version, or any part of it. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it internally without ever releasing it outside the organization. But if you release the modified version to the public in some way, the GPL requires you to make the modified source code available to the program's users, under the GPL. Thus, the GPL gives permission to release the modified program in certain ways, and not in other ways; but the decision of whether to release it is up to you.