By default the copied image is placed top left if the canvas size is bigger. HyReZ is probably right in his inference it is some animation format issue but if it was a still image I'd suspect you'd either just used Canvas Resize with the image tethered to the top left or Copied the GIF onto a new, larger canvas.
If the GIF and white background are in one layer just create another white layer underneath to fill in the now transparent area where you moved it from. In either case: you can try simply selecting the GIF area and recentring or dragging the GIF with the Move Selected Pixels tool to the centre.
Is the animated GIF in another layer to the white background or did you copy the GIF to a new white background image?