All 3 Uses of

sober

in

The Great Gatsby

I've been drunk for about a week now, and I thought it might sober me up to sit in a library.
p. 46.4 *

sober = make less drunk

The hall was at present occupied by two deplorably sober men and their highly indignant wives.
p. 51.9

sober = not under the influence of alcohol (or very serious)

He was employed in a vague personal capacity — while he remained with Cody he was in turn steward, mate, skipper, secretary, and even jailor, for Dan Cody sober knew what lavish doings Dan Cody drunk might soon be about, and he provided for such contingencies by reposing more and more trust in Gatsby.
p. 100.4 *

sober = not under the influence of alcohol

Definitions:

(1) (sober as in: Talk to me when your sober.)
not under the influence of alcohol
(2) (sober up as in: I need to sober up.)
to become less drunk or intoxicated
(3) (sobering as in: a sobering thought)
serious or calm (not silly or excited); or making one serious or less excited
(4) (meaning too rare to warrant focus)
More rarely, sober can refer to one who is rational rather than under the influence of extreme emotion. Also more rarely, it can reference that which is practical rather than fanciful -- such as "a more sober plan."