The Welsh ran in 12 tries as they recorded their biggest ever winning margin at the World Cup, defeating their opponents 81-7.
However, Gatland was disappointed that it took his side until the second half to claim the bonus point, which has lifted them to second in Pool D.
"We started off with 22 points and then the guys felt the game was all over, went to sleep and stopped being aggressive," Gatland said after the match.
"They got a bit of a rollicking at half-time and came out in the second half and did the job."
Victory over Fiji next weekend will see Wales qualify for the quarter-finals.