How to enable 24bit true color support in Gnome terminal?

According to this page, Gnome Terminal supports 24bit true color. This is consistent with Wikipedia.

When I run color-spaces.pl in Ubuntu 14.04 LTS with Gnome Terminal 3.6.2, the result is the following:

enter image description here

While there is no reference image, I imagine that those green strangely-shaped objects at the bottom indicate that the terminal is unable to print 24bit colors correctly.

How to enable 24bit support?

24bit support is enabled by default but gnome-terminal has to be in version linked against libvte >= 0.36 (as stated on the page you mentioned). Which unfortunetly is not the case in the latest ubuntu 14.04 (at the time of writing).

As a workaround you may try:

sudo add-apt-repository ppa:gnome3-team/gnome3-staging
sudo apt-get update
sudo apt-get install gnome-terminal
sudo add-apt-repository -r ppa:gnome3-team/gnome3-staging

Last step removes their ppa - it's optional but be carefull, there is whole gnome3 at that ppa.

Oh and the screen: