Smart Garens would Q at you whenever you try to bully him and this is where mana becomes an issue. As Kayle, you can either W for movespeed to kite back or Q him for the slow, either of which results in Kayle going oom very soon if done a couple of times. Also, like 70% of Garen players are fond of sitting in the lane bushes waiting to ambush you with their Q+E which rewards them with massive damage. This can be pretty hard to counter if they do it right.
Perhaps with something like Glacial Augment, she might be able to bully him. Other than that, I don't see it.
Another way is to rush Nashor's first item, with no boots or tier 1 boots. If he goes for a Cleaver cleaver start(Phage, Kindle etc) or any damage item, Kayle can soundly defeat him in lane. However, all he has to do is build Cowl. Some Garens even start Spirit Visage. These purchases shut that shit down very quickly.
Another problem with the match up less related to laning is his roam potential. He is far more likely to get something out of a roam than Kayle is in the early game due to how low her damage is compared to his, especially after level 6. Kayle's ultimate is kitchen appliance, his is a nuke.