The intended chain line for the SRAM Omnium crank is 44.5mm. No spacers should be used with the Omnium, as the spindle length is not designed for the wider width of a larger bottom bracket shell. The spacers create a wider shell from 68mm, to 73mm, when using each 2.5mm spacer on each BB cup for the GXP BB: this is intended for mountain bike cranks ONLY, which require the use of a 73mm BB shell for the longer MTB spindle. The Omnium used with a GXP threaded bottom bracket is designed to work with a 68mm shell only. Adding spacers moves the bearings outward, which ultimately will bind your crank on the drive side spider.
See our Frame Fit specifications to be sure this crank fits your frame can be found here.