The X can be removed by making a new blank layer, and making two lines that cover the x make sure they are black, and change the blend mode to overlay should work. The text can't be removed, just crop it out.
To me, it looks like you CROPPED the photo and adjusted the GAMMA only. I can do the same without GIMP. I was hoping that GIMP would allow me to REMOVE the watermark, not just GAMMA it to fade.
I did like I said, made a couple of lines, played around with the opacity and blend modes. I also did a cloning job on the suit since the lines were very visible on it.
Glad to help out. It was harder then I thought since the image is not all one color, while certain areas would be invisible with say, 30% opacity, other areas of the line was even more visible.