Its not open sourced. But it's not at all based on adobe Photoshop. I guess if adobe was desperate they could try to sue for a similar UI. But, it's very different than modern Photoshop and basically just closed sourced GIMP (but still free) with a favor to having a similar UI to classic Photoshop.

If it was a daily driver. I'd probably prefer GIMP. But for a friendly free photo editor when I need it; there is nothing more convenient.