The Rotate 180 degrees command rotates the active layer by 180° around the center of the layer, with no loss of pixel data. The shape of the layer is not altered, but the rotation may cause the layer to extend beyond the bounds of the image. This is allowed in GIMP and it does not mean that the layer is cropped. However, you will not be able to see the parts which extend beyond the boundary of the image unless you resize the image canvas or move the layer.