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Yes, but neither of them know nor acknowledge about this relation. As far as they're both concerned, Broadway is a child of the entire clan.
The idea that one of the gargoyle eggs from Wyvern was the child of Goliath and Demona came up way back in the first episode. The idea for their daughter and the name Angela came up early in the second season planning. The decision to add her was primarily to have a female in the cast, though other benefits (a mate for one of the Trio, the connection with Demona) didn't occur until some time later.
I think she's happy for both her father and Elisa. Elisa's more like an older sister than a mother or step-mother figure, but keep in mind that Angela met Goliath and Elisa at the same time. That relationship was firmly established in Angela's mind long before she knew Demona was her mother.
Angela and Gabriel saw each other as siblings- there was no romantic relationship between them. Gabriel is actually mated with Ophelia.
Brooklyn was drawn to Angela as little other than an attractive and available female gargoyle. Broadway, however, saw the person she was beneath that. Angela also saw the person Broadway was, and the two connected.
It's a chicken and the egg thing.
Did the Coldtrio's unbound spirits sense something or cause something?
At any rate, behind the scenes, we certainly used it as a stepping stone to an eventual Broadway/Angela relationship.
Yes, three of them- Artus, Gwenyvere, and Lancelot. Further, their grandchild or great-grandchild is Samson, the lead in "Gargoyles 2198."
They'll probably lean toward the Gargoyle Way (children of the clan, not individuals).
GargWiki.net has answers for all your Gargoyles questions.
Includes episode commentaries by co-creator Greg Weisman, interviews with the cast, and a documentary on the fan convention.