This is one of two setup missions for the smart approach on the jewel store heist and consists on getting some gas grenades that are in the back of a Humane Labs van. Now although you may do the setup mission by any order, and actually the Bugstar Equipment shows up first, in order to make these missions and keep this unique livery Boxville, you must do this mission first, because after both setup missions are completed you start immediately the heist mission, making it impossible to store the van at that point. To get a gold medal along with the van it's easier to kill the driver and before entering the van shoot the back doors and grab the grenades, because once you enter the van, the game assumes that you are going to deliver the van instead of the grenades and thus making you fail the Loose Cargo requirement.
Although it is considered as a story mission this one won't add a percentage for the 100%, although it's mandatory in order to complete the game, thus getting 100%
Note that usually you get a wanted level when attacking the van, for some reason this time it didn't happen, so keep in mind as when you get hold of the van you may need to loose the cops.