Hola amiga ¡Piedra Libre! ; por el contexto pudiera referirse a el grito utilizado en el juego infantil de las escondidas cuando encuentra a alguien. Esta es utilizada en Argentina. En Venezuela este mismo juego a las escondidas, le llamamos loco escondido. Lo jugaba cuando era niña.

"Piedra libre" is an expression that's used in a game, in Latin America (Argentina). It's like hide and seek. It consists on a child who's counting down, and the rest hide themselves. If one of them gets to "the wall (the stone)" where the child was making the countdown before him, he screams "piedra libre", and he doesn't have to be the one who counts down the next round. So, regarding that text, I think that's an expression to say that the narrator was expecting something that could put an end to that search, so he/she could find what he/she was looking for, in that case, "la versión adulta de Gideon Mariette".