Let's see if this helps. Over here we have Essex girl jokes, like most jokes that insult a particular demographic, they're not very funny, and I'm not here to support them.
The protagonist of Essex girl jokes is stupid, vain, shallow and over sexed. They're nearly always called Sharon or Tracey. Sharon and Tracey are stereotypical Essex girl names.
There's a reality programme over here called The Only Way is Essex
, (TOWIE). This deals with real life people in Essex. Like most reality shows it's a load of crap, and I don't watch it, but certain people from TOWIE have made it on to other panel/comedy shows. One of theses is a lady called Gemma Collins.
Gemma is a common name in Essex, but it's not stereotypical, you wouldn't use it when telling an Essex girl joke like you would Sharon or Tracey.