Gnar actually has a mediocre laning phase after his recent nerfs. He has one of the shortest ranges for all ranged top laners early on.
His melee form is actually a point of weakness against certain champions, like darius.
He doesnt outscale darius's tank because he only becomes really tanky in mega form, and his current meta build forces him to go cleaver mallet which has very little in terms of resistances. Darius has incredible base stats and can do damage with just cleaver alone, leaving room for a sixth dedicated tank item. He also has innate sustain which gnar doesnt have.
Gnar outscales him in terms of cc, thats without a doubt.