I like the file manager "Thunar".
Is it available for GNOME as well?
Where/how can I download it?
Most likely its available in your distro's repositories - search your distro's package management system. Which distro are you using?
Ubuntu, CentOS and Solaris
sudo apt-get install thunar
...or search for it in the Software Centre.
sudo yum install thunar
No idea for solaris.
Note: keep on eye on the dependencies that are going to be installed alongside of thunar and make sure you are okay with them.