Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Items and Spells
CDR, Health/health regen, and Speed
essentially it's a tanky gnar that can spam abilities
CDR is an amazing stat to max on gnar that synergies with his entire kit allowing him to poke and escape more often as mini gnar and lay out a punishing CC train as mega gnar. note that max CDR puts gnar's ult on a 48 second cool down at max rank and gnar has a 41 second minimum delay between transformation meaning you will have your ult up every time you transform
Defensive items let you stay in the fight and dishout damage as mega gnar and importantly you can rely on you w passive to compensate for a lack of AD in the build (which is also ofset by the increased casting frequency from CDR). Although this build offers only a bit of MR and Armor the health regen makes up for gnar's poor sustain. (with visage and warmogs the health regen in mega gnar is so large that he gets 20-40 health back every tick)
A word about movement speed
Why ghost over flash? Movement speed in 6.9 is at a high premium since the removal of boot enchantments. Ghost work's really well with Gnar's innate speed boost from hyper and when coupled with swifty boots and dead mans it allows for fast re positioning/ initiate/ escapes.
TP -simply build up your rage bar until you're about to transform --> TP--> hop --> ult -->double kill