What are some alternatives?
When comparing GIMP and Sketch, you can also consider the following products
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Pixelmator - Pixelmator is an image-editing application for Mac and iPad.
Affinity Designer - Professional creative software, exclusively for Mac.
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.