It's about as easy as getting from Gatwick to Stansted (which is to say: not very easy at all). Assuming you're not prepared to spend $200 on a chopper then the train at around $30 is probably your best bet - I'd go
AirTrain (JFK-Jamaica),
LIRR (Jamaica-Penn Central),
NJTransit (Penn-Newark). This gives you 'only' three changes (including the airport shuttle at EWR), which may make a difference if you've a lot of luggage.
Out of the rush hours you can try a cab - I just hope you don't get the driver I had who (a) didn't know the way to Newark and (b) was only capable of effective communication in French and Haitian Creole
