Thanks for your question. You should be able to find all of the information you need on our visas for Belgium page.
Being a part of the Schengen zone, you'll need a schengen visa to travel to Belgium. Make sure that you have all of the required documentatio, including a letter of invitation from your girlfriend.
Keep in mind that visa regulations can change suddenly, so I'd suggest visiting either the Belgian embassy nearest you or their website to get the most up-to-date and accurate information.
All the best and I hope you guys can be reunited soon!