Well, two prices really. One for him and one for the Council.
For the Council it was a promise that Yoda feels bound by now- that they would never tell anyone the truth of what actually happened to Gallagher's emotional state at that time. Yoda does not want you to make presumptions based off of that secrecy, but he hopes you understand how he feels bound not to tell., Revealing to others, aside form being unfair on Gallagher, would cause issues.
Secondly. Gallagher responded to the incident as many in shock would- hazy and jumbled recollections of a highly emotional nature. His treatment... encouraged that imprecise memory. Gallagher has been trained to reject any attempt to remember how he felt at that time. This repression of memory is of course dangerous, but it was the only alterantive- Gallagher was not ready to truly come to terms with what happened.
Now, the Council did not intend this state of affairs to last forever. They felt Gallagher needed to mature and gain experience first- which is why he was eventually allowed on a mission under heavy supervision.
They considered bringing him in to talk of these issues after Droben, but felt such an adrenaline rush of a combat mission was not enough, especially as Gallagher was already comign to terms with having killed his first man. So they thought if he did one more rather more thoughtful misison, after that would have been the time for him to truly come to emotional terms with his past. That mission was Spiridos, and you may remember how 'suggestive' the Council was that Gallagher had a capable bodyguard of a Jedi to look after him.
"So... our problem, I hope you see," says Yoda. That plan has been intercepted in the most unfortunate way- a coincidence if you like, a plan if you are paranoid; Yoda feels it is simply destiny, and a grace challenge. Gallagher, at his most vuilnerable point, has been taken by this 'Sienar'- and now his coming to terms of those events will not be dictated by the Jedi Temple, but by this Sienar.
If Sienar can convince Gallagher that at that time Gallagher did actually fall prey to his emotions and let them dominate him... then Gallagher will be in grave danger.