The plot is kept up by spit (I don't understand, for example, how the outlaws discover if and when Powell will lead a transport, as he only tells the gold owner (Moorhead), who infact I thought was involved in the hold-up. But she isn't. Also it isn't showed what Powell planned to do to discover who the outlaws are, as he is instead easily robbed, his pard killed and he knocked out. When regaining senses though he manages to kill one of the outlaws paassing there (why?) with the money: that is a long call.
I assumed it was Greer who tipped off her men about the gold shipment. It was Moorhead's gold but Powell was using Greer's stagecoach, right? If it's not too much of a stretch to figure that Greer knew Powell would be using her stagecoach at night, then it's not too much of a stretch to figure that she realized it was for gold (what else?) and tipped off her men to do the robbery.
But I definitely agree with you that it makes no sense why, after the robbery and Powell wakes up – presumably the next morning or afternoon – he comes across one of the outlaws transporting the gold.
Also, doesn't it seem mighty strange that the outlaws would have allowed Powell - a newbie whom they suspect – to transport $50K in gold? No way would they let that out of their hands.