Most Helpful Guy
He probltt just smiled because you were looking at him. As thats a nice thing to do (its not like he would give an angry face), there dosent really need to be any emotion behind the smile. It was just a smile...
Most Helpful Girl
Maybe he thinks you're cute. Maybe he's nervous because some strange girl was staring at him. Only one way to find out. Say hi to him next time. Worst case scenario, he's not interested. Best case, you get a cute gym guy.