James: I started as an Argosy Navigator. Drop timing was controlled by the Nav lying on a board up front, looking down through an aiming window in the nose. He would have calculated a release point, though I have no idea what the Forward Throw for your "Gas Bomb" might have been, and established it in terms of yards left/right and long/short from the desired impact point (IP). Having no idea what your scenario is, I can't say what DZ markings there might be to assist in estimating ground distances. Maybe just a smoke canister on the IP? On the run-in to the Drop Zone (DZ), the Nav would advise the pilot with Left/Right calls and, with 10 seconds to release, would call "Red On" and a red light by the rear para doors would come on. Over the release point, he'd then call "Green On" - a green light would come on and the load release would commence.
I'm assuming that your "Gas Bomb" would have been rigged something like a standard 1-Ton Container - ie pallet mounted, on guide rails, and probably with an small extractor chute fixed to the upper clamshell door. At "Green On" the extractor would hopefully drop free and open in the airflow, pulling the load out and initiating main chute opening clear of the aircraft. Sorry, I can't recall detail of the braking system on the guide rails that would be released, or of the rigging system that was used for main chute initiation - that was Loadmaster and Air Dispatcher business, and it's all 50 years ago. (Dougie M may be able to assist if he sees this - he was another Nav involved in airdropping on the Argosy and C-130 for much longer than I was.)
Hope that helps a little.