You're out of luck reaching Crete on OW metal. You can get to ATH from the US via LHR (3x), BUD (2x), or MAD (3x). Most direct would be JFK-LHR-ATH, then JFK-MAD-ATH.

Another option is JFK-AMM-ATH but that's the long way.

Thessaloniki can be reached from LGW if that in any way would be more useful than ATH. IB was planning to start a service to Corfu but again, not too helpful in reaching Crete either.