The Nordica GPS is a softer version of the Grand Prix. This boot is a production version of the World Cup. I used to ski this boot. I think the sizing and foot types are pretty consistent. My shoe size is an 8 1/2 or 9. My feet measure 26cm. I used a 25.5 size. Fit is for a relatively narrow heel, narrow-medium width fore-foot, with fairly low instep. I know guys with size 10 and 11 shoes who were in 26 shells with some grinding.
I'm a size 10 1/2 and use the 28 shell in the Grand Prix R (it has the same last). The liner has compacted enough that I might be able to fit into the 27 shell but I'm not having any problem with the 28. The boot runs normal width in the forefoot and fairly snug around the ankle. If you have low calves the top of the boot might be uncomfortable.