Gardner showed tremendous physical gifts at Sierra College in 2011, catching 61 passes for 1,099 yards and 13 touchdowns, but he was out of football last season after academic issues kept him from joining the University of Oklahoma.
He also reportedly was arrested twice over the past couple of years, once for possession of alcohol as a minor and the other for obstructing a peace officer.
The Dolphins decided to give him a shot, but with the understanding he has to stay out of trouble.
"They just loved the raw athletic ability I have," Gardner said Wednesday. "They told me to come in and really fight for a roster spot. They know I can come in and make a lot of big plays for them. That's what I want to do. They were strict that I have no leeway. I've been in some trouble and I told them I'm going to try my best to stay out of trouble. Everything is behind me now.
"Just moving on and trying to get my career started."
The Dolphins appear set with their top three receiver spots with Mike Wallace, Brian Hartline and Brandon Gibson, but there should be a lot of competition for the final two or three spots (depending on whether Miami keeps five or six wide receivers on the 53-man roster).
"He's a big guy, catches the ball with his hands well," Coach Joe Philbin said. "He's an intriguing prospect. Obviously we've got to get our hands around him and teach him our system as quickly as possible and see how much he can absorb and learn, transfer it from the meeting room to the practice field, and then see how he does in competitive situations."
Gardner, who says he runs around a 4.5 40-yard dash and currently weighs about 225, says he likes watching bigger receivers like himself, such as Julio Jones, Dwayne Bowe and Calvin Johnson.
Gardner also said he would have joined them as a first-round draft pick had he been able to play at Oklahoma last fall. As it was, Gardner went undrafted.
"No doubt about that," Gardner said. "I most definitely would have been a first-rounder. No doubt about that in my head."