Hill was given a one year contract - I don't know if that was his choice, thinking he could get one more 2 year contract next year, or the Rams' choice. But they will need to either resign him, promote Gilbert, or sign another #2 next season.
It wouldn't shock me if they were hoping somebody would fall to them earlier in the draft, somebody more likely to be ready next season, hence the short contract to Hill. In any case, a decision will need to be made after the season. But until then Hill is the clear #2 - Gilbert is an intriguing but very raw developmental guy, who is not even guaranteed a roster spot yet.