I had the same problem where the folder seemed to have the owner of root. I couldn't change the chmod properties of the folder to 777 (it was stuck at 755).
Here's a solution if you're stuck at step 4 of the Gallery 2.0 installation process:
1) Delete the folder [yourdomain]galleryg2data that was created by the installation process.
2) Recreate that folder you just deleted.
3) Do the chmod command on that folder.
When re-creating the folder, it is created under your user login so you will have permissions to carry out a chmod.
Hope this helps.