Jobs was "inspired" by a visit at Xerox labs, they showed him a GUI built using Smalltalk (which they'd also invented). So naturally, he ran back to his office and invented GUI ;)
Or for third I guess lol Jobs demoed a mac gui to Gates, and apparently Gates ran strait to his Microsoft office, where he too invented gui. Jobs was very upset for years.
I think it actually was an Apple innovation, at least for {hobbyist, home, personal} computers. I did some digging and wasn't able to find anything before the Apple II+ in 1979. Please do prove me wrong, though!
Did they? AFAIK, Apple always used “Plus”, not “+” (see https://en.wikipedia.org/wiki/Apple_II_Plus, https://mirrors.apple2.org.za/ftp.apple.asimov.net/documenta...), and “+” is shorthand invented by the community.
The Macintosh Plus, similarly, wasn’t a Macintosh+ in Apple’s marketing, AFAIK.
And, looking at https://en.m.wikipedia.org/wiki/Amiga_500#Amiga_500_Plus, it doesn’t look like Commodore stuck + on the end of their model names, either.