Ok, so it's not built into OS X Mountain Lion 10.8 to view them?
No it is built in. I dunno why the user above wants you to install something.
1) They play automatically when selected in column view
2) You can select a gif in any view and press the space bar, the quicklook window will pop out and loop the gif
3) You can open the gif in any modern web browser
4) Quicktime '7' can play gif files
5) You can open the gif in preview and hold the left or right arrow to move through the frames of the gif.