Can I hide the title bar of MPlayer in gnome?

If it's just for MPlayer you could use the -noborder argument.

mplayer -noborder my_video.ogg

As a more general option, if you're using Compiz (default), you can:

1. Install and launch the CompizConfig Settings Manager
2. Select the Window Decoration plugin and hit the "plus" sign next to "Decoration Windows".
3. Describe the type of window you do not want title bars on with an "And" relationship, and mark "Invert". For example:

Click "Add" and enjoy!

One can remove the border of all windows, it is however a little hacky. I described the process a bit at:

http://grumbel.blogspot.com/2011/06/forcing-fullscreen-in-linux-for-apps.html

And have some code floating around at:

https://github.com/Grumbel/fullscreen-tools

Using the code from the git repository you can do:

./windowtool.py --decorations 0 ACTIVE

Which will deactivate the border, using 1 instead of 0 will reactivate it. ACTIVE refers to the window that has currently the focus, you can also give the window id which you can obtain via:

wmctrl -l

PS: The code isn't pretty and the README in there is a bit out of date.