Rozier is a 22 year old two way PG on a rookie deal signed through 2019-2020. After the 2019-2020 season he becomes a RFA and the Celtics will be able to match any deal that comes his way, basically they can control him for the next 7 years or so. Celtic front office as well as teammates have made a point to talk about the big improvements Rozier has made to his game, he also had a really good SL. Rozier looks like he's made a big jump in his game, the defense was there last season and now it looks like he is more confident in his offense.
Gay doesn't make much sense for the Celtics. They currently have Jae Crowder playing SF and just drafted Jaylen Brown with the 3rd overall pick. Mortgaging a young 2 way PG for Rudy Gay to come off the bench just doesn't make sense. FWIW Rozier has been the most hyped Celtics of the offseason.
The main reason why Rudy Gay would make sense for the Celtics is that he can create offense on his own. The Playoffs showed, that the Celtics lacked in that regard last season. Horford is a nice player, but he is not a guy, who creates a ton on his own, leaving the Celtics once again with IT as the focal point of the offense and with Bradley as their second best player, when it comes to creating some offense.
Given that Smart is a candidate for big minutes at the guard spots, Rozier is a bit of a luxury. Basically just one of those guys can be the main backup PG, forcing the other to play most of his minutes off the ball, where Smarts lack of a consistent jumpshot can really hurt the team. Of course Stevens will opt for a lot of three guard lineups and with Smart's bulky built he can certainly play good minutes as an undersized SF, but is that really an ideal solution?
Now when it comes to the forward spot, it's crystal clear, that Crowder and Gay are more than capable to play both forward positions. Against most teams it won't be a problem to play Gay alongside Crowder and it would certainly be a much stronger forward tandem as something like Amir/Crowder, Jerebko/Crowder or Olynyk/Crowder.
So basically I would judge the Celtics as one of the very few decent landing spots for Rudy Gay.
But I as a Kings fan would certainly prefer to get Marcus Smart in a trade over Terry Rozier, even though I noticed his SL performance and was impressed by his contribution in limited minutes last season.
I would expect Stevens to prefer Rozier over Smart, because Rozier provides good enough grittyness on D and is the better shooter of those two.
I personally would go with Smart for the Kings, because we are in desperate need of some real defensive playmaker, while we could compensate Smarts lack of shooting prowess with lineups incorporating Barnes and Casspi at the forward spots.