You can just watch it, or you can play around with the frames per second variable - if you set it to a negative number the video plays in reverse. The slider only goes from -50 to 50, but if you want a lower or higher value you can click the "custom fps value" button and then you can type in any fps you want.
I was just wondering what would happen if I added a GIF as a sprite to a scratch project, so I made this one using a video I took a while ago. I apologize for the low quality.
