dboy,
GCSE's are, or were when I took them, secondary school exams that you usually take at about 16 years old. They are the first major educational grades in England, After that you go on to do "A" levels a couple of years later, and then some will carry on and do a university degree.
2 minutes on Google suggests to me that your equivalent, although forgive me if you are from the other part of Belgium, is Certificat d'Enseignement Secondaire du Deuxième Degré.
Either way, you will probably need a higher level of education than this, which I'm sure you already have.
DW