I was going to reply “I doubt they will save much more than a minute in total in GMC”, then I found this:
Holy. Fuck. Nuts. This practically skips 11 levels that took me about 6 minutes to complete in one go with descent speedrunning pace and godmode enabled. This doesen’t even screw up the weapons. The skip happens between 0:11 and 0:16, I included the rest to better show at which point in the game I am and to show that you can get your hands on the chemgun fast afterwards (since the skip starts in the level you normally would get it).