Seth Smith is starting in RF or DH vs all right handed starters. Trumbo is starting in RF or DH vs everyone (except for an off day obviously). Gentry only made the opening day roster because they didn't need a 4th or 5th starter. He'll probably get sent down once Rickard is activated from the DL. He's just the 4th OF.
Kim has only played LF. He's not very good out there but he's not horrible. Buck doesn't seem to really like him though. Last year it took a full month of hitting .350 for him to finally start getting playing time. I think it's because he refused the minor league assignment out of Spring Training but that's just my opinion.
TL:DR: Mancini is fighting Kim for starts vs right handers in LF and should start in LF, RF, or DH vs all left handers.