All we know of Santi/Gabi:
She had a giant portrait of both of them, whether it was their wedding or someone else's, who knows?
He likely trained/scouted her as evidenced in her drawer pictures
She had an argument with someone who couldn't take five minutes at the track meet to say hello (Santi was there)
Said if they don't care, why should she before boning MC
Next time you visit her house, the portrait is gone
She is out reluctantly celebrating with Ximena, possibly because she is single
One thing we can rule out is them being family, because MC tapped it.