Gnar's "w" passive has a good amount of damage for being able to hit someone three consecutive times. In his suggested build, we see he only has to build regular boots because his base speed can outrun many champions, especially with the "w" passive's speed buff when having successfully hit a target three times. Building off this allows for you to deal heavy amounts of damage to opponents early game as well as be able to get out quickly after having dealt a ton of damage with three hits in addition to Grasp of the Undying's ability that comes with three hits as well.

Pros:
1. Movement speed will allow for quick escapes and easy targeting, making some plays forgiving whereas some end in death.
2. Opponents will have a difficult time having to resist a large burst of magic and physical damage.

Cons:
1. Late game may be a challenge if there is a large number of magic resisting opponents. Try building duskblade instead of Warmogs if you're confident you can evade enough to successfully trade.