It's impossible for Garen to start with 100 AD in the first place. (Only old Kayle could do this and that involved getting Rally)
The max amount of AD you can possibly begin with is an extra 25 from runes and masteries. This means that if you take a Doran's Blade at level 1, the most you can start with is 85 AD at level 1. The bonus damage being 35, thus resulting in 306 AD max damage you can do at level 1 not counting armour.
This is in comparison with his Q, which, using your format, will do 297.5 damage.
However, there are two things you are not considering.
1. Decisive Strike is vital to Judgement actually doing anything. Without a silence and a speedup (especially considering this build has no boots), you will practically NEVER proc all 3 hits off judgement. Literally never. Not even if Alistar knocks someone up and you were spinning the whole time will you get all three off. There's so many reasons for this. The enemy is not silenced and can flash / ghost / blink away. Without the speed boost, everyone can outrun the spin by the third proc. Because the enemy is not CC'ed himself, he will just CC you instead. If the enemy is Xin Zhao, you'll just die. Etc. Etc.
Thus, practically speaking, Judgement will only ever do at most 2 procs at level 1.
This means that at most, Judgement will only ever do 204 damage at level 1. The exceptions being if you have chain CC or a level 1 teamfight. This is the important bit.
2. Getting hit by a nuke that removes half your health at level 1 in an instant has a faaaaaaaaaaaaaar bigger psychological effect than gradual damage over time.
Tl;dr: You will never proc judgement 3 times without silence + speedup. Thus Q will always do more damage.