How do I make a GUI to cover the whole screen when a part is touched?

I want to make it so when a player touches a part (for example, a portal); I want a GUI to pop-up onto the entire screen to make the screen fade to white or black, and then after a few seconds, it fades and disappears. How do I do this?

Try the Humanoid.Touched event, and do this all client-side. Ignore touched requests if there is already a GUI animation in progress:

http://wiki.roblox.com/index.php?title=API:Class/Humanoid/Touched

Make GUI visible, use a loop or tween the transparency, and make it disappear.