I kinda see it as maybe a little bit of both, though primarily Gabriel wants the three of them together simply because he knows that now Sam belongs to Castiel and so they are a package deal. But at the same time he wants the same soul/grace bond they have because it's something he's never been able to have and he's been cut off from everything for so long. Simply put, he's lonely and he wants in on their relationship. Cas is uncertain, distrusting, but Sam wants to give it a shot since he kinda relates with Gabriel and he talks Castiel into it because Cas does want to help his brother.

Did that make any sense? 'Cause I feel like I just went in circles. XD