I do know what I am saying and doing.
Timboli: I'm sorry, but that makes no sense.
What is it exactly that they are obfuscating?
GOG throws all files into a blender, so if you unpack it with innoextract (without) the special switch or with innounp, you will get a big fat {tmp} folder with many many itty bitty chunks, but not the game itself. In case of Terraria 1.4.2.0 you will end with about 14000 chunks (files) there.
These chunks will be reassembled during installation by a special compiled Pascal script (I nearly reverse engineered it fully out of frustration, until I got the memo with the switch). This is also the main reason of the disk thrashing, disk wearing and delay in the installation, because this compiled script has to reassemble the files.
There is a reason, why this switch exists with innoextract, it does that transparently for you.
Timboli: And I think you are mixing up InnoSetup with InnoExtract.
No. InnoSetup is the installer system used by GOG; InnoExtract can extract the data and support files from the executable installer; but discards the embedded script. Innounp cannot deal with the provided garbage, but does not discard the embedded script and all the other "invisible" stuff if you want to have it.
Timboli: And that --gog switch does not do what you are thinking it does.
If you don't want to install the game and just want to extract the data, then you can do that, --gog switch or not, though that might be needed for the content of any BIN files associated to the EXE file. Though the BIN files can be extracted with UnRAR or 7-Zip in any case.
I may quote the homepage of InnoExtract (I said the same, but in more layman's terms:
-- snip --
Other newer GOG.com installers don't include the raw files directly but instead store them in GOG Galaxy format: split into small parts which are then individually compressed. These files are named after their MD5 hash and stored in the tmp directory, for example "tmp/ab/d7/abd72c0dddc45f2ce6098ce3a286066a". innoextract 1.7 or newer will automatically re-assemble these parts and extract the original files unless the --no-gog-galaxy option is used.
-- snip --
Timboli: In reality though, just installing the game extracts everything.
Yes, if you are using Windows.
If you are using Linux, you really don't wanna fire up a WINE environment or a full-blown VM just to install the game. Prior to the addition to this nonsense, it was true, that you could decompress the installer with RAR or 7z, but if you do this with the "newer" ones, you end with a mountain of chunks.
For reference with Terraria 1.4.2.0
installed size: 494 MB
files: ~ 13500
GOG Original installer Size: 418 MB
files: ~ 13000 files (tmp folder is alone 13008)
installation time (excluded redistributable): 2m 22s
GOG rebuilt installer - adjusted the setup.iss to accomodate the original files and threw out the custom code by GOG
- lzma2 solid maximum compression
Size: 395 MB
files: same as installation, plus some script files and picture files
installation time (excluded redistributable): 42s
Granted, most time the original installer is busy decompressing chunks and reassembling them for not really anything valuable.