I think she was married(currently no longer married) to someone else and had an affair with Santiago.We don't have a proof that it is Santiago, we just saw a big Picture of a Wedding, we know she is married, but that is no proof at all, It might be Santiago, or just another random man.
And It doesn't add up, Santiago is married since a few yrs to Adriana to a point where her daughter, call him Dad instead of Stepdad or use just the first name when refering to him.
Santiago was her agent coming out of high school, not sure when their relationship started.
Gabi hoped Santiago would ditch his family to be with her, but this never happened.