Use the canisters that explode and drop one 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 a while, but it works.

Throw a few canisters at the van without shooting them. This works best when at mid field. When you have them in front of the van, throw 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 few more and it is done.