how do i open 2nd fence on babylon level? after part where truck brings you inside.

First, shoot all enemies running around there except the one above. Next, run into the room behind the two trucks, use the crate to climb the upper level and get the treasure. Now find the stairs, go down there (the last enemy waits there for you), whip across the gap and climb the ledge. Now you are inside the structure (more enemies around) where you should find a lever to open the gates you mentioned. Good Luck!