The GMAT is part of your application, however ESSEC takes your whole profile into consideration. If you are very good in one area it might compensate for a weakness in another area. Therefore, a good GMAT Score might be an advantage but no guarantee to get into the program as well as a less-than-perfect GMAT score is no knock-out criterium. In any case you should aim for as high a GMAT score as you can possibly achieve