Peter it's an issue that I have never really looked at.
Some users have issues, some don't, I don't. But the Gemini FAQs#3 is a good starting point. This has good advice and a link to another article on ground loops.
There have even been threads
in IIS about this. specifically with
this as a solution.
Usually, from my limited experience in this area, the ground loops issue is usually cause by one PSU being "not right" and using one PSU for all is a better "thing to do". Maybe use the above tests to find the culprit PSU.
edit:
I'll add...you shouldn't get a ground loop thru RS232 on Gemini if using a Gemini BT device (that takes pwr from Gemini).