If my friend had an account here, he could tell you better. But in a nutshell, it was a very slick and very fast Pre-Emptive Multitasking OS for the Atari ST line of computers.
I do not recall how compatible was with software written for Gem or what ever, but I do know that it had a nice selections of apps already written for it which ran amazingly fast!
Nearly everything was in German, but with a Hex Editor, you were often able to go through and translate the programs (as well as the OS itself) into English.
As a side bonus, they ported the project over to work on the 68k Mac line of computers, though you could only launch it from within Mac OS. Amazingly enough, it still flew!!! I remember stripping down Mac OS down to the bare basics and setting this OS up to auto launch during boot time. One thing I remember is that I was able to go on-line with it, and I often forgot that Mac OS was running underneath because the speed increase is SO HUGE!