ME: Given that the Rangers will be still in a growth stage in 2019, I think it makes the most sense for Guzman to get more opportunity at first base with Gallo remaining in left field. Defensively, it gives the Rangers above-average fielders at both spots. Offensively, they both still have work to do, but that's the luxury of a rebuilding team: It can afford to commit at-bats to guys who have work to do.
The wild card in this is Willie Calhoun, but I see Calhoun as more long-term DH than outfielder unless he makes significant steps as a fielder.