plus, she made it clear all along she would "do anything" to improve her position, literally too I'm sure
Gannicus had made it clear he owed no allegiance to any side, he's just making decisions as a free man. He had no motivation to steal the map. According to wiki, Gannicus is part of the rebellion but historians don't know for sure if all of them were led by Spartacus or had their own following - conventional wisdom says Spartacus was leader.
Later this season, we may yet see Gannicus join the band, at least I hope so
The trio of S, C & G would be formidable!