Gallup and others like Newsweek, NBC tracking polls, CBS, ABC, NYT and WP all did great at predicting the 2016 results:
"It’s not over ‘til it’s over, V.2: Yes, Trump might be happy about one poll that puts him one point ahead, but perhaps he should take a look at this Moody’s Analytics model. The model, which has correctly predicted the winner of every presidential race since 1980, predicts that Clinton will win next week. More specifically, the model forecasts that she’ll take 332 electoral votes, while Trump will receive 206. Swing states Ohio, Pennsylvania, Colorado and Florida are all set for Democratic victories, according to the model, which is based on three economic factors and three political factors.
Remember: Just seven days to go until the election."
Truth be told they just had no real clue and probably won't in 2020 either ... jmho