SOLVED
Its trying to install to my system hard drive that is only 100mb, and I can't find where to change where it installs.

I've included picture of the steps in the installer before it tries to install.

It shouldn't be that hard to change where you want to install your application.

A 100 MegaByte HDD with Windows 10 installed (12 Gigabyte footprint) doesn't sound right at all. Are you sure you're not confusing MB with GB (ie, it's a 128GB SSD or something)? Or the normal Windows Partition with that "System Reserved" 100MB Boot Loader partition that Windows creates (the one that should be invisible and not even have a drive letter, but can be seen in Admin Tools -> Computer Management -> Disk Management)?

I don't use Galaxy myself, but all of GOG's offline installers let you choose install location per game (eg, if you have a very small C drive SSD and a very large D drive HDD and want to put games on the latter).

It doesn't have windows on that drive. It used to be a school computer, might be some leftover from those days.

But ill try the offline installer

Solved it with your help.

The 100 mb was a partition created for some school purpose like I said.
So I deleted that and changed the letter of my external Hard drive to D.

And it workd