I've written an article a little over three weeks ago describing my experience while developing my first fully native GTK3 app for Linux, with tips and resources on how you can get started developing your own apps yourself for the elementary OS Linux distribution, so this is an update to that story :)
I'm happy to report that my app Copy Paste Grab, a graphical user interface for the popular youtube-dl command line application that lets you download videos from hundreds of websites, was accepted by the elementary OS team and is now available in their AppCenter. I've already had my first three "sales" (it's really donations since paying is optional, the "price" of $3 is merely a suggestion). The idea here behind the app store is to financially support both app developers and the elementary developers, they get a 30% cut of every "sale". Monetizing Open Source work has always been a hard issue and this is one idea on how to solve the problem at least partially, by encouraging users to donate.
The AppCenter combines a typical package management GUI that you can find in all Linux distributions with a "pay what you want (including free)" store for apps specifically made for the elementary OS Linux distribution, preferably written in Vala, a Gnome programming language that compiles to C and resembles C# or Python with the optional syntax called "Genie".
You can find Copy Paste Grab in the AppCenter, or check out the GPL3 licensed source code on Github.
I do plan to make it available for other Linux distributions as well in the future, probably as an AppImage, but in the short term it will be exclusive to elementary OS. The great thing about targeting elementary OS first is that it's a lot easier than the complex landscape of hundreds of Linux distributions out there, each with their own little problems that can affect how your app looks and works, which is horrible for people new to Linux development.
So I can only wholeheartedly recommend trying it out with elementary OS first to have an easier start into the Open Source Linux development world. Some great apps have done just that and later become part of the larger Linux ecosystem, like FeedReader which I've written about just two days ago.