Deal 7% more damage to low health champions. Gain AD/AP after kills or assists.
Gnar loves to build Frozen Mallet and this rune allows you to bypass the item while still getting the same benefits. Hit an auto-attack and chase down your enemy proccing as many [W]'s as possible.
Perfect Timing serves double duty in both providing a clutch 1-time Zhonya's active as well as saving you gold on purchasing Gargoyle's Stoneplate.
This loadout focuses on lane dominance and Future's Market allows you to hit your item power-spike sooner so you can abuse the enemy laner and push your lead.
Approach Velocity is actually a very valuable and abusable rune when paired with Glacial Augment. Glacial's slow paired with Approach's Movement Speed will let you hit land more auto attacks than otherwise possible
Gnar loves a lot of attack speed to abuse his range and proc his [W] as often as possible in melee matchups. With Glacial Augment these rune becomes even more powerful.
WhyCoup De Grace
Between Press the Attack and repeated procs of Gnar's [W] you should be able to quickly whittle the enemy champion to 40% health and get the bonus damage to kill them.