I think his strengths are strong enough that he can't be buffed without making him overly frustrating, but if he gave up some power in the areas that make him annoying then he could get greater strengths in other areas.
To reduce his annoyance factor in top lane and free up power budget, he could have Mini-Gnar attack range reduced to 350 (Urgot level) at all levels instead of scaling up, he could have his Mini-Gnar W damage changed to physical and the movement speed reduced. Gnar would still be able to poke decently well with his Q, but that offers counterplay because it is a skill shot, and his shorter basic attack range would open up the possibility for melee retaliation/aggression when he pokes or tries to free farm.
In exchange, he could get considerably stronger base defenses and base AD for both Mini-Gnar and Mega-Gnar, and he could get to spend more time overall as Mega-Gnar (see R change below).
His Mini-Gnar R could be restricted to use only when at least 50% rage, and it could be changed so that casting it causes Mini-Gnar to leap high in the air and instantly gain 50% rage, transforming into Mega-Gnar while airborne and then pile-driving back down, damaging and stunning targets in the area for 1 second. This would form an interesting engage combo when used with his other leap. The cooldown on his R could be reduced at early ranks also, allowing him to transform even more often. This would give Gnar more control of his transformations lategame and would open up more opportunities to engage teamfights as Mega-Gnar. His increased base defense scaling would help him survive on the front-line more effectively, which is fitting because his reduced attack range will also require him to position farther towards the front-line also. He'll fit better into team compositions which rely on top lane providing a beefy front-liner for proper teamfight formation.