This is why Rasmussen (which got the 2016 presidential election dead on) has such a differently number (49% approval). Its poll is likely voters.
Gallup's poll of just "adults," not likely voters or even registered voters, is a waste of time, unless you're a media looking to write a giddy anti-Trump article.
Yeah, Gallup just polls adults....which is pretty worthless.
Anyhow, today Gallup has Trump at 41% approval.