I believe GEES program's objective is to increase a person's 'employability' in future. Putting this 12 months working period into your resume would be more attractive to future employer instead of just indicating you are fresh grad.

Even if you excelled outstandingly during GEES, your superior still can't take you in as permanent staff unless coincidentally there's an open position at that time. If there's no open position, the superior has to justify/present at so many levels just to take you in.

If I were you, I'd try to get more information on which Petronas company you were offered. E.g. design, refinery operations, exploration, etc. Find out if is this your field of interest.