Once they offer you a position and you accept, you probably should stop applying for other positions for a while. Until that happens you are free to apply to anything you like.
Yeah. He may decide he likes the gatekeeper job better because of the nature of the job. It would involve greeting people, answering questions and other.
At the same time he may decide, after spending time talking to the lifeguards, that isn't something he wants.
If a lifeguard position comes open, he's a known entity, and if he's been a good employee, he would definately be in the running.
I'm not sure if I've seen any of the gatekeepers promoted to lifeguard. I think they have, but I'm not transitioning the faces in my mind from seeing someone at the gate, to someone up on the stand. I know most of the people that work at the pool, but honestly wouldn't notice if a gatekeeper became a lifeguard. Probably because my interactions with the GK is usually about 5 seconds.
However, over the years, I've seen a good number of lifeguards being promoted to head lifeguard, and also other positions higher up in the parks and rec dept in aquatics.
Of course most lifeguards are college students, so they move onto something else after a season, or several years.