purplebass

You mean a gif in a circle? If so, select all the layers of the gif and put them in a group (select > all layers; layer > group layers). Now choose the shape you want to "give" to your gif, in this case you need the ellipse tool. Draw the circle on the top of the gif and rasterize this layer. Next add a layer mask to the group of layers you've created before (third button on the layers palette, or layer > layer mask > reveal all). Get back on your rasterized circle and with the magic wand select outside the circle, then get back on the white layer mask of your gif group (be careful to select the mask and not the group or it'll seem it won't work) and with a back brush delete the part you've selected earlier - you'll see you will delete the parts of the gif are not in the circle. Delete the circle and that's it.