In the latest installation of the "Merlin Dialogues," Julia explains why she thinks Merlin is really Arthur's true love rather than Gwen. The accompanying poll suggests that most readers agree with her.
I think what makes this a difficult question is that there are multiple ways to define "love." Anyone who has read the excellent book, The Four Loves, can appreciate the complexity of the word. Arthur may truly love many things: Merlin, Gwen, his horse, his father, Camelot, Morgana, bacon and eggs... but he loves them all in different ways. They are all "true loves" of his.
In this particular episode, "Sweet Dreams," I believe it required a specific type of love to break the spell on Arthur. The spell caused Arthur to become infatuated with Vivian. Let's call that type of love "eros." So the only way for this spell to be broken was through true love's kiss. I would argue that the type of love needs to match in order for it to break the spell. Arthur's love for Merlin is a brotherly kind of love - it's the love you see between best friends. Let's call that type of love "philia." Arthur's love for Gwen, on the other hand, is that eros love - it's an infatuation, it's passion and desire. Therefore, in order to break the spell - which caused Arthur to feel eros love for Vivian - he needed to be kissed by someone he felt true eros love for - i.e., Gwen.
In short, I'm arguing that Merlin may be Arthur's true philia love, while Gwen is his true eros love. In this situation, it took true eros to break the spell of false eros.