the kid will have a promising future, and from looking at the list of college dmen that stayed 3 years in college then going pro, the future does look bright.
Look at Ryan Mcdonagh, Steven Kampfer (all 4 years), Jeff Petry, Jamie McBain, Justin Braun, and Kevin Shattenkirk, all of these kids stayed patient (minimum three years) in college, and now all are looking to be high end players as rookies. Pretty amazing crop if you ask me. Throw in Mark Fayne (all 4 years) and Ian Cole, and the list gets even more impressive from players who stuck it out in the NCAA on the defensive side.
It seems like staying to your junior is the "in" thing for top end college dmen, so I would assume Gardiner turns pro and makes it to the NHL fairly quickly like the kids listed above have done.