For many years, GIMP didn’t have native macOS support (you needed X11 to make it work). Seashore wasn’t as fully featured as GIMP, but it was native. As of Sierra (macOS 10.12), Seashore just crashes when you try to launch it. R.I.P., Seashore.
Fortunately, if you’re looking for a free alternative to Photoshop, GIMP is still around, and it’s now native to macOS.