If anything, they wanted to test how effective the gas was in eliminating a large segment of the population without the damage done by bombs. Besides, chemical warheads on missiles would be an effective attack on Israeli cities.
Given the level of technology and fabrication skill shown in the bomb I suspect any “missile” built by the same people would have a range of about 1 mile, and land pretty much anywhere in a 1 mile circle from the launch point.
You would get a nuke in response.
It has long been US doctrine that ANY chemical attack on US forces, let alone mass-casualty chemical attack on US cities, would be responded to with nuclear weapons against the attacker.
I suspect Israel's doctrine is the same. I suspect Iran knows this.