The visitors conceded 15 penalties at the Aviva Stadium in what was their first away defeat in the Six Nations since 2011.
"We weren't good enough today," Gatland told reporters. "We weren't at the races. We will take a long, hard look at oursleves.
"This was probably one of the most disappointing performances I've been involved with as Welsh coach.
"We won't be looking forward at whether we can win the Six Nations, we'll be making sure the performance improves against France."
Defeat leaves Wales fourth in the table after two games.