All the gold's safely through the gates of King's Landing. We need to get the last of these wagons over the Blackwater Rush before nightfall. The head of the line is ambushed, the tail will never be able to reinforce in time.
Randyll Tarly
The first scene of packing gold and Highgarden (when they decided to stay back to collect food) was days before the battle. It must have taken a while for Dany to get the intel/raven before she left via ships & dragon. The Lannister forces were intercepted at Roseroad, close to wherever they cross Blackwater Rush river to reach King’s landing. This is very close to King’s landing.
The gold reached King’s landing along with the food stolen from Highgarden
The food which Lannister forces were stealing from farmers was ambushed, they didn’t know by whom but it’s safe to assume the Dothraki took it.
Wagons at the battlefield were the tail end, 100 at best, which is one day’s worth of food supply that Highgarden used to send to King’s Landing.
I hope this helps if you didn’t understand offscreen travel, logistics, communication, what commodity belongs to whom and what really was the damage in the battle - a day’s worth of food and majority of Lannister army.
It was an excellent military strategy to destroy their forces with minimal damage and a great way to lay groundwork for the siege on King’s Landing. Dany followed Tyrion’s plan with the addition of destroying a large portion of the Lannister army.
