Sorry for taking so long to respond. I tried attaching the file, but GOG seems to only allow pictures to be attached to posts. If someone wants a more straightforward solution instead of asking support, I would suggest that if you have other Sierra games from that same time period, do a search for the INSTALL.HLP file in their respective folders. I noticed that Gabriel Knight and Quest for Glory VGA seem to include essentially the same file.
As long as the file is not too big, you can rename the "appendage" of it from .hlp to .jpg or .png, and then attach it to the message. You know, fooling GOG to think it is a picture, while it is not.
By default stupid Windows hides those file types, so you have to enable it from File Explorer options before you can see them, and change them.
I actually have the original Red Baron floppy version (bought back in the early 90s) which is installed in my DOSBox, and I did earlier check it to get that file. The problem is, at least my floppy version doesn't have that INSTALL.HLP file. Not sure what the GOG version is based on, some CD-ROM version? There is INSTALL.COM which is normally used for configuring the audio options. but I am not sure to which file it saves the configuration.
I didn't think to check other Sierra (Dynamix?) games though, I didn't know the same file can be used from other games.