It's under the filters > map menu. But results are a bit hit and miss. Sometimes what I do is create a 200% x 200% tiled image, manually fix any obvious seams and then copy each side I changed back to the original image, which is a bit fiddly and time consuming but can give better results. I don't know a better way to do it in GIMP but there might be one, or maybe a plugin you can download.