The Rotate 90° counter-clockwise command rotates the selected layers by 90° counter-clockwise around the center of the layers, with no loss of pixel data.

The shape of the layers is not altered, but the rotation may cause the layers 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 layers.