trigger_gravity allows you to change gravity force affecting players (players only, not projectiles). Note that it will set the gravity to whatever is specified in the entity and to reset it to previous strength you need another trigger_gravity.
As for gas. If its supposed to hurt people , make a trigger_hurt volume in an area, set appropriate damage, then add a func_dustcloud or func_smokevolume with appropriate particle color.
That would be hard, assuming you want to target a specific person who grabs the suit. It would have to be some sort of plugin, and treat it like an individual entity like the briefcase.
Could be possible with 32 booleans indicating whether or not a certain person has the suit, and check and update that stuff, but I'm not sure it is possible actually.
You could fire an "AddOutput" input to the !activator that touches a trigger where the suit would be to set the player's targetname to something like "immune" and have a filter_activator_name on the trigger hurt that only allow players not named "immune" to be hurt by the gas.
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