How do kangaroos gather food?

Kangaroos do not gather food (gathering implies that they collect it and store it to eat later) they are herbivores and they graze on grass and other green vegetation.

Kangaroos only live in areas where there is sufficient vegetation such as grasses and young plant shoots. They are notorious for eating the tops off young trees, where the shoots are fresh and green.

Similarly, the reason one sees so many dead roos by the side of the road in the bush and the outback is because they graze by the side of the road and, when they see a vehicle, have an unfortunate tendency to hop away - in front of the vehicle. The grass at the side of the road is usually fresher and greener, getting some water runoff from the road and condensation arising as the warmth of the road meets the cold night air.