On the GWT website itself, there's a link that you can download the zip package:
code.google.com/eclipse/docs/install-from-zip.html

But since it's probably filtered in Iran and Google doesnt allow access, you need to try something different:

1. download and get a proxy like Ultra Surf first.
2. Download a Download Manager, like Internet Download Manager and set it up to use Ultra Surf in its proxy settings.
3. Then, using the Internet Download Manager you should be able to fully download the zip file.

The reason I suggest using download managers like IDM, is that they support resume functionality, and VPN/Proxy programs often tend to drop the connection in the middle, which tends to not work well with eclipse  and can seriously screw things up. With IDM, when Ultra Surf drops the connection for example, you can simply reconnect and resume the download instead of starting from the beginning.