Carrara won't render a .gif with a transparent background. The usual trick is to make a colored background close to the background that the gif will be residing on  and then delete the background color frame by frame in a Photoshop or gifmaker. 

That said, transparent .gif files are always problematic because there will always be a "fringe" color around the objects unless you don't mind a truly pixelated edge. Gif files are soooooooo 1990's. :D

Mark