Willie Walker kicked two first-half penalties for the lead and he increased it to nine points early in the second half.
But then it all went wrong as Jacques Deen, Jean-Baptiste Peyras (two), and Vincent Inigo scored tries, with Daniel Larrecha adding two penalties and two conversions.
Ryan (left) had predicted the game would be a lot more intense than last week’s trip to Canada and he was right.
He told the club website: "For 50 minutes we were organised and played within the structure we had talked about. Our 9-0 lead could have been grown out even further but we then made two or three silly errors, missed a tackle here and there and our reaction to that was panicky.
"We had to try and create things we had not worked on and when you do that in France you get picked off. For 50 minutes it was pleasing but there can be no excuses for mistakes, whether you are a youngster in this environment for the first time or an experienced international.
"When you play for Gloucester it is about operating at a high level consistently and in that respect we should have been more competitive. But Bayonne are considerably further down the line than us in terms of preparation and we have never moved away from the fact that pre-season is about trying new players and new combinations.
"I would never offer that as an excuse because there are things to work on but we can move on from this and prepare for our next game."