The GPL does not restrict how you may use the software, it only places conditions how you can distribute or modify the software. Thus, using a GPL-covered IDE to produce proprietary software is perfectly fine. Similarly, it is well-established that using GPL-covered compilers (like GCC) to compile proprietary software is perfectly fine. An editor, IDE, or compiler only processes your software as data, but does not insert GPL-covered material into your software that could cause your software to be subject to the GPL. There could be issues around included templates, snippets, or macros, but those are usually exempted from the license. As far as I can tell, Notepad++ does not have a built-in snippet library.
Allowing commercial use is not unique to the GPL. All open source licenses allow use for any purpose, including purposes like creating proprietary software. The “freedom to run the program as you wish, for any purpose” is the foundation of the Software Freedom concept. Similarly, the Open Source Definition forbids “Discrimination Against Fields of Endeavor” (OSD #6).
