How to kill Harriet with poison gas
The way I did this was with the Shift slab with 'Swapper' upgrade. Sneak in under the Hanger 2 and climb up behind her. Kill the eternalists first, then use swapper to get Harriet into the side room with gas in the bottom and simply kick her in. Get out of line of sight quickly just in case she tries shooting (which will blow up the gas) but she should die quickly.
She fires the Gas 6-Pounder so the trinket that allows you to breathe gas could be handy too but I didn't find I needed it.
I found it hard to do this with the Gas 6-Pounder gun, because when she shoots back it will explode. However kicking her down over and over while she chokes is possible.