This looks like Aseprite. Aseprite is already open source and you can get it for free, all completely legal. The only caveat is that you need to compile it yourself (which takes 2-5 shell commands). I think this is more than fair, but ripping off Aseprite is not so much. Their license also strictly prohibits that behavior.
I even did a write up. It was one of the first reverse engineerings I've did. https://gist.github.com/Throvn/97fcb4981c1ff66725d4b2e408ba0...