Better yet, don't itemize for damage on gnar. Mini gnar has enough base damage and kiting utility to not require any, and mega gnar is a tank first and foremost. All these people rushing botrk irk me to no end when I see it.
At worst, rush a trinity. You shouldn't need the sustain of botrk because you outrange almost every top laner out there, you should never be in a situation requiring the active because you have slows and a jump in mini and a jump, stun, slow, and knockback stun in mega, botrk is standard rush for an aa dependant champ when gnar is a spell weaving champ in both forms (the q cd on catch justifies this claim), and it leaves you more vulnerable during the build path than trinity force for less utility and tankiness.
Botrk sucks copious amounts of dick for how big a trap it is for most champions.