"A token gesture" means "an insignificant act that doesn’t make a real difference." In the context, someone pours out some alcohol, apparently, but he or she did not really do anything to get rid of all of the alcohol because there is more in the desk. The action of pouring out the alcohol is perhaps insignificant if that person really wants to stop drinking.

A "token" can be "a symbol." In some board games, where many players move around the board with a recognizable game piece for each player, the pieces are called "tokens." Although its so old-fashioned as to be a joke, someone trying to be courtly might call a gift "a token of my esteem," a symbol of my friendship. By extension a "token" can be a meaningless symbol--something done half-heartedly, for show, in place of doing something real. For example, if a company had 100 vice presidents, all belonging to group X, and they hired just one person from group Y to be a vice president so they could pretend they were not discriminating, that Y would be "the token Y." Another expression that means something similar is "to give lip service." That means to say you are doing the right thing but not actually do much.