that ring is where

end1.sky wrote: DomeModel()
{
Geometry("end_sky_dome_trees");
Offset(95.0);
MovementScale(0.992);
}

is. It isn't a gap in the texture, it's where that model goes. If you don't want the (very lowrez) endor trees in the skydome, delete that line. if you do want it, you should replace the ones in your map's folder with the ones from the endor folder and
A. go into the data_ABC/_BUILD/World folder and delete the munged file
or
B. update the time stamp using correct programs (msh- hex editor, NOT text editor, tga- image editor, .option- text editor)

Yes, it works! Thanks for every bodies help, especially Commander_fett. Thanks.