Changing GIMP 2 installation folder on Windows

On GIMP 2.8.22 at least, the "Customize" button is on the bottom left of the installer:

Easy to miss.

At least with the GIMP 2.10.x installer, using CMD to do gimp-<VERSION>-setup.exe /? revealed that the installer can be started with an install-directory pre-defined. Presumably this is for people who are doing regression-testing or such and want multiple versions, but this can also solve your issue.

For the installer I just downloaded, this worked for me, which I typed into CMD after cd /ding to the folder where I downloaded the installer-file:

gimp-<VERSION>-setup.exe "/DIR=C:\Program Files\GIMP 2.10"

In my case, it was trying to install to "GIMP 2.8", which was not what I wanted, especially since I was trying to install 2.10.x and the name would have been misleading.

You could use GIMP portable, and copy it into another folder on a different partition.