SRAM GXP crank installation and rear hub preload
By amazing co-incidence there happens to be a video on YouTube showing SRAM GXP crank removal and installation on a Bike Trek Superfly 6. 


                
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Referring to that video, on the non-drive side it's (from inboard to outboard): red rubber seal, black plastic spacer, metal wavy washer. On the drive side there is only the red rubber seal.

Looks like your bike shop has omitted the black spacer on the drive side.