it's sounding like to me Gamboa may have signed what they referred to as a "customarily bout agreement." I don't know how legally bounding that is.
This is from The Ring:
It’s common in professional boxing for the official contracts of a bout to remain unsigned until long after the matchup has been announced, sometimes until the week of the fight. However, the fighters customarily sign a bout agreement that details basic terms of the deal – including how much they are to be paid.
Still Gamboa had plenty of time to express his displeasure with his contract and he certainly didn't have to wait till the press conference.