Generally cloning Git repos over HTTP protocol is known to be very slow and unreliable if there HTTP proxies involved, etc. I am also not sure about EGit plugin (I operate 'bare' Git via command line - it works wonders for me)
So I'd recommend installing Git on your system and cloning Grails Git repo on the command line via 'git' remoting protocol: 'git clone git://
github.com/grails/grails.git'
Works fine for me.
Cheers,
Dmitriy.