That's a weird one.
I try to keep a stack of 99 Gyshal greens on me at all times - there doesn't appear to be any downtime between the Chocobo dying (or being dismissed/running out of time) and me being able to resummon him via using more greens.
It's not an action/ability that gets used, it's literally just dragging the Gyshal greens items from your inventory to your hotbar and using it like 'food'.
The first set of Greens you get whilst doing this quest are a key item, but the rest are just standard inventory items (you get given one set at the end of the quest, any subsequent sets you need to buy from vendors or gather)
Is there some kind of error message you're getting when you use the greens? Might need to just /bug it and see what support come back with...
