In Belgium you are free to choose your specialist, which are either located within a hospital or at a separate location. Be aware that you always have to make an appointment in advance and sometimes you have to wait a few weeks or months before an appointment is available. You can find information here: http://www.hospitals.be.
Your GP will also be able to advise you on specialists and hospitals but you do not necessarily need an official referral from a GP to see a specialist. However, if you see a specialist without a referral, your costs may not be reimbursed or you could be reimbursed at a much lower rate. Also, getting an appointment through your GP may make it possible to get an appointment sooner than when you make the appointment directly.