Is GIMP the best open source alternative to Photoshop?

Well... yeah. Compliance with open source eula is far easier than expensive software licenses

For photo editing with minimal effort I would also check out the Nik Collection. Google recently acquired the company and released the software free to the public. https://www.google.com/nikcollection/

I would take a look at the tablet versions of photoshop they are just as robust and the software is free. Several tablets also support pen and wacom inputs which can be extremely beneficial if you are planning on using this for professional applications.