That's a Crankcase Ventilation
hose. It connects to the driver's side valve cover.
Two part numbers for 1st or 2nd design.
12617139 is 1st Design
12624774 is 2nd Design
Model year 2009.5 is the 2nd Design, and 2008-2009 is 1st design. I cannot vouch for the difference between the two, so buy whichever matches what you have.
'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
"If you want to criticize someone else's work, be sure that your own is in order first."