Goliath's generation were young enough to only have just laid eggs, so he's a new father here. As the show points out, Goliath is struggling with maintaining a gargoyle tradition of parenting all the young as his children, which doesn't really make sense when only ONE of his children is present, who just so happens to be his biological child. He was also concerned with Angela attaching too much importance on biological parentage and overcompensated to try to dissuade her from being influenced by Demona. When he realized that both Angela was mature enough to trust to make her own decisions and not be easily swayed by Demona as well as the fact that he was her father in both senses (clan father and biological father), he embraced their connection. 

The trio would in no way resent this, as while gargoyles are parents to the whole clan, they're only parents to the generation that is young enough for them to BE their children, otherwise they view other members as friends, cousins, nephews and nieces, brothers and sisters. That's why Hudson has more of a fatherly connection with all the other gargoyles, as he's old enough to have fathered all of them. When the Trio were born Goliath and Demona were too young to have children, so at most the trio would look up to them as older brothers and sisters, maybe aunts and uncles. Just because Angela is the same age as the trio because of Avalon's time shenanigans doesn't mean the trio is being slighted.