I think you are getting the usual UK avionics shop "expertise" there
The Insight unit is quite popular in the USA, though not as popular as the much more marketed JPI one, and in Europe GEM never made much progress (compared to JPI) presumably due to the very strong avionics distributor channel brand loyalty.
There are no general reliability issues on the GEM. Have you called their US head office? They are said to be responsive.
The CHT probes should last "for ever" because they don't get very hot. EGT probes are something else; they will always fail eventually (due to erosion by the hot gases, until the metal is eaten all the way through) but JPI ones fail much sooner because they are thinner-wall. IME the JPI ones last a few years.
If you had multiple channels show a fault, that would be an instrument issue, possibly on the connector but much more likely internal.
Earthing is a redherring; the probes are thermocouple probes which go all the way back to the instrument and there is no earthing involved. The power -ve terminal goes to the airframe but if this was missing it would not work in the first place
In terms of connections, these instruments are fairly simple; you have the power and then you have a bundle of thermocouple probes whose 2-core cables (made of special t/c wire) go straight back to the instrument.
One can get probe faults caused by bad connections within the probe leads, which usually have a joint somewhere; the probe comes with maybe a 2ft lead and this has to be joined to a t/c cable which then goes to the instrument. These joints can be flimsy if done badly, and a lot of avionics wiring is dry-crimped which is a recipe for bad joints - but this would not explain all 6 going.
You may just have had a random failure - the usual causes tend to be corrosion (damp over the years) or vibration.
I would deal with the US company always. There is no warranty now so dealing with anybody in the UK is pointless.
A JPI unit won't be any better. I've seen the inside of an EDM700 and the build quality is not great.