Leaf Node Monitoring is licensed under the GPLv3. The source code however, is not publicly available. You will receive the code alongside the executable with your download. You are of course free to do whatever you want after receiving the source code as long as the GPLv3 license allows it. I do not intend to make money (profit) with Leaf Node, just to cover the running costs (hosting, domain, payment provider, possibly a Qt license if it takes off).
I do want the source to be available. I’m not against publishing the code, if a customer does buy the program and publishes the source, it’s their right to do so and I’m fine with that. Just a little bit of a hurdle to get people to pay a small amount, to cover the running costs. No intent to make a profit.
The GPL license FAQ has three Q&A items regarding charging and source distribution which answer all the questions you might have:
Does the GPL allow me to sell copies of the program for money?
Yes, the GPL allows everyone to do this. The right to sell copies is part of the definition of free software. Except in one special situation, there is no limit on what price you can charge. (The one exception is the required written offer to provide source code that must accompany binary-only release.)
Does the GPL license allow me to charge a fee for downloading the program from my distribution site?
Yes. You can charge any fee you wish for distributing a copy of the program. Under GPLv2, if you distribute binaries by download, you must provide “equivalent access” to download the source–therefore, the fee to download source may not be greater than the fee to download the binary. If the binaries being distributed are licensed under the GPLv3, then you must offer equivalent access to the source code in the same way through the same place at no further charge.
If I distribute GPLed software for a fee, am I required to also make it available to the public without a charge?
No. However, if someone pays your fee and gets a copy, the GPL license gives them the freedom to release it to the public, with or without a fee. For example, someone could pay your fee, and then put her copy on a web site for the general public.
The last line of the second item, “you must offer equivalent access to the source code in the same way through the same place at no further charge”, seems to be covered as far as I can tell when I provide the source together with the download.
As a bonus, the app itself has the source code embedded inside itself, which you can save to disk at any time via the about screen. This is mostly to be able to ship the source alongside the app on mobile platforms (appstores) where we don’t have the ability to email the user the source code. If you’re interested in the technical side of embedding the source code in your Qt app, see the guide here.