What TR said...
And is GC under the impression that you are seeing only him? Have you explicitly, in words which were said aloud, agreed to be monogamous with him? If so, you need to address that as soon as possible.
Is that why you think GC would be hurt that you have other lovers? Or other reasons?
Finally, do your other involvements know about GC? If they don't, it would be a courtesy to tell them, especially if the relationship with GC evolves into something serious. I'm juggling a marriage and two lovers and have found it very helpful that everyone knows about everyone else.