While there is no official policy on this, so you'll never get an official answer, trust me that V will clear before G. V is for elite members. G, while used for systemwide-type upgrades, is also used for anybody using some SkyMiles to upgrade (i.e., many non-elites), so it is going to take a back seat in the pecking order to upgrades for elites.

You can see this is you look in advance at flghts with both G and V seats available. There are almost always more V seats than G, in my experience.