If you sign and change your mind, you lose two years of eligibility and have to sit for two years.
If you change your mind AND the school you signed with grants you a release, you sit a year/lost a year like any transfer would.
The NCAA can also eliminate that penalty by granting a waiver (which they don't seem opposed to doing for recruits whose coaches have left).
I've never heard of a school letting a guy out of an LOI without a coaching change or other issues surrounding the school/player, so can't envision a scenario in which Gonzaga willingly parts ways with him.