Here are the exact gel polish colors I've found to do an every-nail-is-a-different-color neutral ombre manicure.
A tip in case you're worried about being that customer at a salon as you set handfuls of nail polish bottles on the table and state that you want to use ALL OF THEM: I now always offer to pay a little bit more when I explain that I'd like every nail a different color. I've only ever had one salon take me up on that offer (they charged me $3 more), but the other manicurists seemed appreciative of the acknowledgment that it does take more time than a standard manicure.
From light to dark:
Thumb: OPI "My Vampire is Buff"
Index: OPI "Did You 'Ear About Van Gogh"
Middle: OPI "Taupe-Less Beach"
Ring: OPI "Krona-Logical Order"
Pinky: OPI "Shh...It's Top Secret"