It's a weapon. If you look at the model in the HLMV it calls it the 'belly cannon'. I'm guessing it was supposed to be the equivilent of the strider's main cannon, but none of the gunship fights in HL2 needed it.
heh, i was just trying to figure out AI lol, its the SDK_Vehicles map, and i binded "[" to npc_create npc_civilian and "]" to npc_create npc_combine_s both with RPG equiped, then did a give npc_combinegunship and quite a few give npc_strider
it was fun to watch, but the only problem i am having is... i need a complete description of the AI nodes, as i don't think i quite understand them, if someone is willing to give me a quikie on this, i would make a full on SP map like this for *hits and giggles to watch with scanners targeting for the strider's main gun etc...
Need explanation & suggested placements pretty pretty please on hints & info nodes