All is not lost! You just might need to back up a step or three and let Gertie get used to the idea.
It's not clear from your post whether the kitten is confined to her own space/room or if she has the run of the house. If she isn't already separated from Gertie, start there. Let your existing cat have the run of the house except for the kitten room, and then go through the steps you saw when you read the introduction threads - feeding on opposite sides of the door, scent-swapping with towels, etc. Lots of people go through "re-introductions" if the cats don't cope well the first go-round. It can take a LOT of time, but it's worth it in the end if both kitties are happy and can at least tolerate each other.
I read a thread some time ago that was really helpful for us when our existing cat (Lando) was showing clear signs of stress and jealousy toward Leia (the kitten). It had to do with making sure the resident cat gets EVERYTHING first. Feeding time? Lando gets the first bowl. Playtime? Lando first. Petting? Lando got first dibs, and if he showed any signs of jealousy while we were attending to Leia, one of us (my husband or myself) would go to him separately. If he wanted to be left alone, we respected that, too. Basically, we tried to make sure that if one of us was shut up with the kitten, the other was playing or just sitting with Lando. Basically, if you can demonstrate to Gertie that you recognize her dominance, then she might feel a bit less unsettled.
I'm sure someone may also mention that Feliway, or a similar product, might help relieve Gertie's stress, which sounds fairly severe.
Hope this helps a little - good luck!