Please keep in mind I'm no expert: I gathered some information about closed source software, but I cannot warrant for completeness or even genuine truth.
GLib, GTK+, Granite and Vala are licensed under the LPGL. There is a TL;DR for this license. When your application is dynamically linked to these libraries (which is default when you build an application for elementary OS / Linux in general), you do not have to disclosure your source code:
Derivative works (including modifications or anything statically linked to the library) can only be redistributed under LGPL, but applications that use the library don't have to be.
While the basic libraries I mentioned (which are necessary for creating an elementary OS app) are licensed under LPGL, others might not. So if you are looking for advanced functionality in additional libraries, you should be really careful about the GPL license. The GPL itself requires you to open your source code for every owner of your application and it "infects" software which is using it. So you'd have to release your app under the GPL as well.
Some GPL-licensed projects do provide a GPL linking exception. So, if in doubt, message the developers and ask about their stance.
One thing you should keep in mind though: Linux users are used to open source software. While the closed source state of some software is accepted / tolerated for various reasons (for example nVidia/AMD drivers or Google Chrome), people might reject the idea of a "simple" application being closed source.
