Just taking a guess here, but from your other posts and their responses I doubt that installing gparted under backtrack is what you want, even if you could do that.
As already noted, backtrack is long since abandoned, which may be why you get the 503 error when attempting to get the package.
But since you seem to want do something with your backtrack partition, you probably cannot do that easily from within backtrack, rather the person suggesting gparted most likely had in mind using a Gparted live CD/DVD.
To make one of those simply go to the Gparted project site
, download the ISO and burn to disk. Then boot the Gparted live disk and manage the partitions from that point.
Finally, if you really want to get an answer about the 503 error, please post the URL which is generating that response so we can see whether it is a package repo or something else.