* Create a text layer in white with the Text tool. It is useful to have the text with explicit spaces at least on one side. You may also want to add an empty line below. Another way is to set the box to "Fixed" n the tool options and enlarge it manually
* Filter > Map > Tile (uncheck Create new image). Give it the size of the image if you keep it straight, make it bigger than the image if you intend to rotate it (up to about 50%, depending on angle)
* Rotate and center the watermark layer with the Rotate tool
* Reduce its opacity
* Crop the image back to its initial size
* Filter > Map > Tile (uncheck Create new image). Give it the size of the image if you keep it straight, make it bigger than the image if you intend to rotate it (up to about 50%, depending on angle)
* Rotate and center the watermark layer with the Rotate tool
* Reduce its opacity
* Crop the image back to its initial size
If you want to watermark several images, you can create the watermark layer as a separate image (large enough to cover all you needs, rotated and with reduced opacity), and then use File > Opens as layers to add to it images.