He can transform at any level, it's his passive. It is not a fully random mechanic. You control whether or not you're going to tip your rage over, and when you do it. From what little I've played, your rage can slowly diminish, not sure if you have to recall, or just sit out of combat for a while.
A great Gnar player will create a situation beneficial to Mega-Gnar while still Mini-Gnar, and use the 15 seconds of Mega to their advantage. Both of his forms are very powerful in their own way, Mini's poke, kiting, and sustained harass; Mega's heavy CC and innate tankiness. They wanted to introduce another transformation champion that didn't fall prey to the same issues that Jayce, Elise, And (old) Nidalee faced: One kit being strongly preferred to the other, so they sit in one all day (And chuck spears). Thus, the forced transformation was born. This is crucial to Gnar because he can't just sit in one form, he's going to need to use both, and build for a happy medium.
Quinn is a good example of a semi-random mechanic. If you're just standing there, it will mark a random target, preferring things with low health to full health. It also prefers to mark any target you've attacked recently. She can also apply this mark at will with her vault. It is not a fully player controlled mechanic, but you can influence it. If it was on the enemy ADC at all times in lane Quinn would have far too much lane presence, as proccing the mark deals a significant amount of damage.
I feel that her range is the only thing keeping her from being an extremely dominant ADC. With her range kept low, she relies on the blind to get in range and do as much damage as possible with as little retaliation as she can manage. Against one target, that's a very viable approach, but with an enemy support likely nearby, this creates a very awkward situation for her to be in. I do feel right now she's a better top lane champion if only because of the dominance of long range (Cait, Trist Kog) and Caster (Ez, Lucian) type ADCs.
