Interesting. The two characters I can't work with through the gateway do indeed have apostrophes in their names, ambi. I hope this is just a temporary issue!
I'd like to add that for me, I can only access 3 of 5 characters in the gateway. The other two do nothing when I click them from the character selection screen. Annoying since one of the two is my current main.
It is very annoying, I agree. The least they could do is tell you which item is the problem. The duplicate item in this case is the mask. So if you opened the moonstone mask in guardian pack first, discard the the mask and you'll be able to open the 3-item moonstone set in the hero pack.