I remember using GEM back in 1986. It was quite good.
Interestingly it looks similar to a Unix Window System I worked on for Siemens around the same time called Collage (I think). This ran on the Siemens Sinix variant of Unix. I wrote a spreadsheet for Collage and there was a word processor. The system ran on the MX2 / X20 mini computers as well as MX500 multiprocessor systems. One model was a dinky little desktop about the size of a small form factor PC and ran using the National Semiconductor 32 bit processor range. It is was a kind of NeXT Pizzabox before its time. The big advantage of Collage was that it didn't crash all the time.
There doesn't seem to be any Wikipage on Collage so I guess it is lost in the midst of time.