How to hide title bar for a specific window?

Title bars / window decorations are usually specific to the window manager in use. GNOME doesn't support a built-in method to launch a window/program without decorations, unlike window managers such as Openbox.

A solution that works within GTK across any window manager is to use GTK's gtk_window_set_decorated(), with more information here.

Here's a non-programmatic solution that's at the X11 level, and needs only the x11-utils package, that's part of almost all distributions :

xprop -name 'Your window name' -format _MOTIF_WM_HINTS 32c -set _MOTIF_WM_HINTS 2

If you omit the -name option, you will have to click the window whose title bar you want removed. You can also specify the window ID instead, with the -id option.

EDIT : My bad, the correct type of the _MOTIF_WM_HINTS property is 32-bit cardinal (i.e. unsigned integer, 32c), not 32-bit integer (32i), I changed the command line accordingly.