The grey market is name given to “where” stocks that trade without solicited quotes or the ability of a market maker to “make a market”.
The reason I say “where” is because it is not so much a place as a name applied to all stocks that can’t publish solicited quotes by market makers.
Most stocks that get sent there via suspension ultimately become illiquid.
Going there is inevitable. There is a way to come back to quoted trading that requires a market maker to submit a Form 211 to FINRA....but being successful is very rare.
And even if a stock manages to do that it takes well over a year in most cases...usually 2-3 years.