The difference in batting ability is about 2/3 that difference in WAR - the other Win and change is due to fielding ability (Davis is a little below average for a 1B, Gallo is a little above average for an OF) and baserunning (Davis below average, Gallo above average).The next worse qualifying batting average in the majors right now is Joey Gallo's .210. However because Joey is putting up a .316 OBP (somehow!) and .500 SLG, his efforts have been worth 2.1 WAR to the Texas Rangers, just a bit better that Davis' current -2.6 WAR....
Gallo has been (overall) an average MLB hitter with average baserunning and average fielding at a corner OF, playing every day.
Davis has been a terrible hitter with poor baserunning and poor fielding at 1B, playing most days.