But a real dressing down from head coach Gatland and assistant Rob Howley saw Wales impress after the break as they added nine further tries to record their highest-ever score in the World Cup.
"We started off with 22 points and then the guys felt the game was all over, went to sleep and stopped being aggressive," was Gatland's assessment of the first period.
"They got a bit of a rollicking at half-time and came out in the second half and did the job."
Scarlets centre Scott Williams scored a hat-trick for the Welsh, with 19-year-old replacement wing George North becoming the youngest try scorer in World Cup history with his brace.
Aled Brew, Toby Faletau, Gethin Jenkins, Jonathan Davies, Lloyd Williams, Lee Byrne and Alun Wyn Jones were the other try scorers, with double Lions tourist Jenkins bursting through a handful of tackles for a memorable individual effort.
Stephen Jones, a Test Lion in both 2005 and 2009, kicked six conversions and a penalty for a 15-point haul on the day that he became the leading appearance maker in Welsh rugby with his 101st cap.
Rhys Priestland added three conversions but Gatland refused to get too carried away with the scoreline as despite there being plenty of positives for Wales to build on.
"The disappointing thing of the night was the scrums were pretty messy, and there were a lot of penalties, but we'll have a look at the tape and sit down over the next couple of days and look at the side for Fiji," added Gatland.
"The impact off the bench was really good and overall, with the number of changes we made for the game - 11 - I'm really pleased.
"We wanted selection headaches in terms of players putting their hands up. That was the challenge to the players - give us something to think about in terms of selection. The players did that."