How do the CC know which GPS are good and which aren't? They DON'T! For that matter, if something is marked 'Garmin', is it really, and not a Chinese rip off?
Well quite, this is what I think on the subject:
Would a decent GPS system made by the main brands, designed and certified for use in aircraft interfere with other avionics?
Answer, No.
Should GPS systems be allowed on board?
Well arguably not...for exactly the reasons the last two posts have cited.