Januzaj left United for La Real earlier this summer in a permanent deal.
Asked about Van Gaal, he told AS: "I do not want to talk about him, because everyone knows there were difficulties with him. We did not have the best relationship, everyone knows. It was not easy, it's very frustrating and difficult when you find something like that."
On Jose Mourinho, the Belgian added: "Nothing bad to say about him. As a coach I have no criticism, from the beginning he was very fair."
Asked who he'll dedicate his first goal for La Real to, Januzaj replied: "Van Gaal. I want to score and dedicate it to him."