Leinster trailed 14-0 midway through the first half but a brace of tries from fly-half Jimmy Gopperth and another from back rower Jordi Murphy saw Leinster move to within a point of a quarter-final place
"To come to a place like this against a well-organised team that haven't given up a lot of tries at home, it's a satisfying result and puts a spring in our step for next week," said Gibbes, whose side host the Ospreys at the RDS on Friday night.
"It wasn't an ideal start. The way they were able to gain access was more about our deficiencies rather than them creating a hell of a lot, but they're a good team, we gave them a little bit and they took those two tries early.
"Castres delivered everything we thought they would - they were massively combative, very physical at the breakdown and they had a good set-piece.
"We just felt that if we were able to get a little bit of a foothold with possession, particularly at set-piece, that we may be able to put pressure on them later on...and it took quite a long time to do that.
"I thought the boys' resilience and energy and their commitment to keep getting up and running into these big fellas here was first class. It took a massive effort from us to score three tries and we almost sniffed out an opportunity for a fourth at the end."
Leinster stay top of Pool 1 with 17 points, with Northampton remaining four points adrift after their win in Swansea this afternoon.
A bonus point victory in Round 6 would push them on to 22 but the fact that they currently have a lower tally than any of the other pool leaders means they are most likely looking at an away quarter-final in early April.
"Of course you want a home quarter and to (be in with a chance to) achieve that we've got to win the game next week," said the impressive Gopperth.
"We've just got to make sure we get the points on the board early, make sure we win first and let the game take care of itself after that. Hopefully we'll get a good outcome from it."
Former Newcastle playmaker Gopperth was outstanding in picking up the Heineken Man of the Match award in France but he admits Leinster will have to up their game to make a fourth European final in a row but he insists their team spirit will stand them in good stead.
"We were disappointed with missing a couple of first-up tackles that got us into that situation (of trailing by two tries). It's the belief in each other and wanting to work hard for each other," added Gopperth.
"Our setpiece took a bit of a hard knock but they got their act into gear and we really got some go-forward ball and looked after it in the second half. That's when we created opportunities and we finished well.
"Leinster have always been a great scrambling team and I think we just worked hard for each other because we want to be in the top eight."