Here's a stone template, straight out of my game files, that has a poison gas effect.
[INORGANIC:PARALYSIS]
[USE_MATERIAL_TEMPLATE:STONE_TEMPLATE]
[STATE_NAME_ADJ:ALL_SOLID:fog of paralysis][DISPLAY_COLOR:3:0:0][TILE:'*']
[SPEC_HEAT:409]
[IGNITE_POINT:11440]
[MELTING_POINT:NONE]
[BOILING_POINT:10000]
[SOLID_DENSITY:1346]
[MAT_FIXED_TEMP:10025]
[IS_STONE]
[STATE_NAME:GAS:constricting fog]
[STATE_NAME:GAS:constricting fog]
[STATE_COLOR:ALL_SOLID:DARK_BLUE]
[STATE_COLOR:LIQUID:DARK_BLUE]
[STATE_COLOR:GAS:DARK_BLUE]
[NO_STONE_STOCKPILE]
[SYNDROME]
[SYN_NAME:paralysis fog poisoning]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_INHALED]
[CE_NUMBNESS:SEV:100:PROB:100:START:0:PEAK:1:END:20]
[CE_NUMBNESS:SEV:25:PROB:100:START:20:PEAK:21:END:50]
[CE_IMPAIR_FUNCTION:SEV:100:PROB:100:BP:BY_TYPE:SIGHT:ALL:START:0:PEAK:1:END:50]
[CE_PARALYSIS:SEV:100:PROB:100:BP:BY_TYPE:GRASP:ALL:BP:BY_TYPE:STANCE:ALL:BP:BY_TYPE:FLIER:ALL:START:0:PEAK:1:END:50]
Just change the syndrome above to yours, and name the thing appropriately. The thing turns to noxious gas after being made/summoned/whatever.
And the reaction to make that, a placeholder reaction, since I don't know what your goal is:
[REACTION:<reaction name>] - change the reaction name as you see fit.
[NAME:xxxx xxxx xxxx] - see above.
[BUILDING:<building here>:CUSTOM_<x>] - change the <building here> into your workshop of choice. The CUSTOM_<x> is a construction shortcut, change the <x> into any key, or set it to NONE if you don't want any shortcuts.
[REAGENT:A:1:STONE:NO_SUBTYPE:STONE:NONE] - change the reagent requirements as you see fit.
[PRODUCT:100:1:STONE:NO_SUBTYPE:STONE:<x>] - match the PARALYSIS with the INORGANIC material's name above.
[SKILL:FORGE_FURNITURE] - change the skill as you see fit.