And with Steve Tandy's men failing to secure a GUINNESS PRO12 win for the third successive match, Gibbes admitted it was the opening stages that cost them an important success in their quest for a semi-final spot.
"It is the old cliché, but it was definitely a game of two halves for us," the New Zealander said.
"The intensity, the attitude and the effort in the second half was what we wanted, but the problem is that we didn't start like that which is what you need to do in these big games.
"There were 20-something errors that we made, and against a team like Leinster, if you give them chances like that they will take the points.
"That is what they did - they took six easy points in the first half based on our errors, so that is disappointing and frustrating for us as a group.
"The disappointing thing for us as a group is that we were in control of a lot of the game, but there were moments where we just needed to do better, particularly in the first half."
The defeat means the Welsh side have not registered a PRO12 victory since a 22-11 triumph over Newport Gwent Dragons in January, a run that leaves them five points adrift of top-of-the-table Glasgow Warriors having played a game more.
But despite that run, Gibbes insists they are still well in contention for the end-of-season knockouts.
"We are still cracking along, you never say never in these things and it is a bit early to start talking about that," he added.
"We have got Munster next week, we have to get that challenge right and get a result out of that, and if we do that then we are in a pretty good place.
"As a group we have a competition that we really want to be in the play-offs for, and although the group is disappointed, we have got a job to do now, so we have to keep moving forward."
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