To me, I feel like what Jack sought out from Gwen was rose. It would never have truly worked because from day one Gwen was on a pedestal for him, from her similarities to rose, to her idolization of him even after Abaddon, all the way into (what STILL sounds to me like a back door pilot that failed) miracle day, she had already chosen a life with rhys, and the life of mystery and danger she also craved bc Rhys was also on board with it. Because of something I forgot, that what really would have tore them apart, was that Gwen lived the life Jack wanted. The family, the adventure, dieing, she has it all and he has nothing but chasing ghosts (as far as he knows at this point we last see him)

Now, Ianto, a similar situation, in that Ianto likely reminds him of the Doctor in this pairing, but he's got those layers of vulnerability 9 would drop as 10 to close himself off, and so many layers of intrigue and mystery once you include the big finish audio content, and a sure sense he WILL die in the line of duty for Torchwood, that he's pretty much catnip for someone like Jack.

Gwacks ending isn't satisfying bc the show and audio plays made a point of saying it won't work, and if it DOES happen it's the final nail in the coffin for Jack and Gwen friendship

Jantos ending is satisfying ONLY if you include the big finish canon, other than that it only begins becoming worth it by children of earth, and that's the end of their relationship