Johnny Hekker has punter's job nailed down, and team's almost never take two punters on their final roster. I say almost never, because I never would have imagined a team using their third-round pick on a punter.
Fisher has been pretty effusive in his praise for Hekker too, making that roster spot a no brainer.
Adding Gill is the more interesting move here because he has an NFL background. The Colts brought Gill into the league as a UDFA in 2009, so Fisher would have seen some of him that year, the only year Gill saw any regular season playing time. He also has a stint with the Eagles, who use a similar defense to the Rams.
I would say that the most likely thing here is adding a defensive tackle to the depth chart, especially with Trevor Laws still dealing with an injury. Adding Gill would keep the reps down along with the injury risk for the other players. Then again, the coaches could be concerned about their depth behind Michael Brockers, Kendall Langford and Laws.