There's nothing wrong with putting the letter G in anything as long as you don't use the word Gravis. The letter itself is perfectly acceptable under fair use.
Anyone remember the unix mail reader program "PINE" as in "Pine Is Not Elm"? That's even more obvious, but still fair use.
The Gravis trademark is dead. So we'd be extra safe just by not spelling out Gravis. So, the use of "G" or even the word "GUS" is, again, fair use and totally safe. I would not use the word "ultrasound" in combination with almost any adjective like "ultrasound ultimate" or any such. Trademark office databse is FULL of actual ultrasound companies (think medical imaging) with nearly every cool adjective used in combination with the word ultrasound. Steer clear.
I still like ARGUS, too... R could be Refinement, Research, or Revision. I like Shock's idea of research... has a nice ring to it... Advanced Research GUS... ARGUS. Advanced Revision doesn't sound bad either (research is better, IMHO).