Is there a GUI for Ubuntu server ?

No there isn't any GUI in Ubuntu server.

But: you can install one if you require.

# To install minimal Gnome core desktop, run this in terminal:

sudo apt-get install xorg gnome-core gnome-system-tools gnome-app-install

# To install Unity GUI desktop, run this in terminal:

sudo apt-get install --no-install-recommends ubuntu-desktop

# To install LXDE (Lubuntu) desktop, run this in terminal:

sudo apt-get install --no-install-recommends lubuntu-desktop

I hope that helps :) 

Of course,

there is ubuntu minimal and ubuntu desktop, but why would you need a desktop if you're going to run port80;443;25 .

Saving CPU time and memory is a big deal in servers.