I read this comment and I felt really really sad of people not reading their champs in depth.
Well, as quick explanation, when Gnar reaches full rage you rage meter will turn red, Gnar will turn red and you will ear a special sound indicating Gnar will enter en Mega mode and it will hapen in the next 3 seconds, that is how the passive is intended to work, when your meter is red, Gnar itself turned to red and you ear the sound indicating Mega form is triggering and you don't want to wait you can cast any ability to inmediately turn into Mega Gnar and the skill casted will be in Mega form (including Ultimate despite it is uncasteable in Mini form). So for clarification terms in the ways Mega Gnaro works:
1.- Get the rage meter full (red meter, red Gnar, special sound) and wait 3 seconds to Gnar to tranform
OR
2.- Get the rage meter full (red meter, red Gnar, special sound) and cast any skill (including Ultimate) to get instant Mega form and a Mega form casted skill.