naarcissus: Workaround:
Use a program such as Advanced Run by NirSoft. Ugh, you'll have to find it yourself, can't post links, I guess. :eyeroll:
In the Advanced Run window:
Add the GOG installer to the "Program to Run:" field.
Set "Run As…" to "Administrator (Force UAC Elevation)" in the dropdown menu. (Advanced Run doesn't have to be run as admin to do this.)
In the "Environment Variables" section, in the dropdown menu, select "Use only the following environment variables (ignore the system variables)"
Click the "Fill Current Environment Strings" button to the right of the dropdown menu. This will fill the text box with variables.
Find "TMP=" and "TEMP=" and change the path on the right of the equals sign to a TEMP folder of your choice. ("D:\TEMP","X:\TEMP", "Path-to-game\TEMP", etc.)
Click Run in the bottom right of the window, the installer should run and use the TEMP folder you specified instead of what is set in Windows.
This doesn't seem actually work. The installer will briefly keep it's own immediate temp files there, but still move the data stream to the usual OS TEMP location.