Centerfield gate opens 2-1/2 hours before game starts, all other gates open 1-1/2 hours before game starts.
For a 7 pm game, Centerfield gate opens at 4:30 pm, all other gates open at 5:30 pm.
Since they give out over 20,000 bobble heads, it is not necessary to get there when the Centerfield gate opens. I would estimate about 2,000 - 3,000 people at most show up to wait for the gates to open (Harper bobble head was pretty crowded).
It is not always orderly, especially when they do not make it clear where the fans should line up at the Centerfield gate. The fans get there early and start lining up, but they do not know for certain which individual gates will be opened until the staff start setting up about 1/2 hour before the gates open.