A bad Gnar build only serves one form. A good one connects ranged lane pressure with transformation impact: enough tempo to punish before engage, enough tankiness so Mega Gnar has time to impose control.
Ruby Crystal
Proc Sheen in Mini Gnar, burst AA empowered W passive
Mega Gnar stacks armor shred with W and R AoE
Shield in Mega Gnar, survival to tank in transform
Plated Steelcaps
Lane sustain Mini Gnar that poke at range and must survive until the transformation.
Magic shield against damage taken during the Mega Gnar transformation.
Reduces enemy punishment in Mega Gnar form after a failed engage.
Accumulated HP to that Mega Gnar is tankier during his teamfights.
Strengthens trades aggressive in Mini Gnar with his boomerangs and jumps.