On my machine, I see innoextract should be probably used with -d, not -I:

> innoextract -v
innoextract 1.9
Extracts installers created by Inno Setup 1.2.10 to 6.0.5

> innoextract --help 2>&1 | grep '\-I'
  -I [ --include ] arg          Extract only files that match this path
 
> innoextract --help 2>&1 | grep 'output-dir'
  -d [ --output-dir ] arg       Extract files into the given directory

So I would suggest:

1. Buy the game from GoG. Don't download their (Windows) installer but the two files the link two (setup_fallout_1.2_(27130).exe - 800K in size and setup_fallout_1.2_(27130)-1.bin - 686M in size from what I've seen).

2. As suggested in the current instructions, brew install innoextract. For me it produced version 1.9 as seen above.

3. innoextract setup_fallout_1.2_\(27130\).exe -d /Applications/Fallout

4. Download the fallout-ce-macos.dmg, open it, pull the Fallout Community Edition app icon into /Applications/Fallout.

5. Running the app will now successfully start the game.