Which gambits have been most successful? Besides the Queen's Gambit (which isn't really a gambit anyway), which gambits have been most successful as far as winning percentage and usage at high levels of play?
According to these statistics from 4 million games, the Marshall Gambit of the Semi-Slav defense is the most successful with 58.4% wins. I only counted gambit openings that had more than 1000 occurrences in the database.

The least successful gambit appears to be the Latvian Gambit with only 41.1% wins.

Note that since the database does not discriminate against older games, the statistics may not necessarily reflect the possible outcome of an opening if it were played today.