-Drop a canisters that explodes on either side of the gate, in the corner between the wall and the gate. Do not throw it. Walk away and shoot the canister. Repeat the process until the van is destroyed. This may take some timebut it works.
-Toss a couple canisters at the van without shooting them. This works best when at mid field. When you have them in front of the van, toss one at the van and wait until the can gets directly in front of the gunner then shoot it. This will take a lot of life out of him. A couple more and it is done.