Had to do the gear cable on my G160S at the weekend, inner and outer as I had managed to wear a hole in the outer somehow, never done an internally routed cable before, hope to never do one again.
Utter nightmare job, impossible to run the outer from the mech end because of the angles on the exit/entrance holes, grommets were so tight it was near impossible to feed the outer over the old inner.
I had to remove the bottom grommet where the brake and gear cables run and was a pain to try and get the new outer back through the hole again as I couldn’t see inside at the right angle even with a torch(although it wasn’t quite as bad as I feared getting that back in again).
In the end I was only able to do it by gorilla taping the old inner to the new outer and push/pulling it through the frame while praying that the cables didn’t come detached from each other. that only worked to a point as I had to remove the tape (removing freshly applied gorilla tape is no fun task either) because the holes on the chain stay were too small, took over 2 hours across 2 evenings.
In retrospect I should have found a better way to attach the old outer to the new outer and push/pull it though that way, serves me right for watching youtube videos on the subject rather than thinking it through myself.