mRemoteNG is written under the GNU GPLv2 license and is a free software.
Free software is referred to freedom, not price. The General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for the service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
To understand and read more of the license that mRemoteNG is using. Please follow the link here to know more: GPL (V2) GNU Project website