I still think this is pretty silly at some level, but if you are determined to do it, here’s a starting point.
Checkout the gradle/gradle repo on github at e09125febb2abd4d5eb70714ff68cdc76ee7dc45
Using jdk6, you can then run:
$> ant dist
This will create a target/dist/gradle-1.0-snapshot directory with a gradle distribution in it. (You may need to create an empty src/samples directory in the repo to make the distribution build.) Even then, it may complain of failing tests, but you’ve got a Gradle binary.
From here, I would attempt to find some later point in the repo where you can build gradle using that gradle binary, and then keep fast-forwarding until you find a version of Gradle which can’t be built with that binary and try building it with the one you built with the first one, and then repeat. Good luck.