Ironically the "grey market" is were all stocks actually start out. Which that definition is geared towards. It is also where stocks listed on foreign exchanges trade if they do not file with the the SEC. ADR's also trade there.
But another subset of stocks which that definition doesn't mention are suspended OTC issues. They go there to die.
They are in a class by themselves,..and don't ever have IPO's. And a great many stay there (>99%) until they are revoked if they are an SEC filer...if they are not they just languish...and perhaps FINRA removes the ticker once it has no action for a few years.
News 
Market Data 
Discover 
