EDIT: To anyone from the future, please see the bottom of this thread, I have attached my solution.
Hello.
Firstly, I am using GTK Radiant 1.6.6, the latest version I can download off https://icculus.org/gtkradiant/.
Secondly, this is how my file path is setup.
Now, these are now textures are showing in the editor.
I have read some other forum posts on the internet, and they were not able to resolve my issue. Could somebody help me with this? Thank you very much.