On my Debian GNU/Linux sid chroot, I have java-gcj-compat installed allowing me to do (thanx man-di!):
JAVA_HOME=/usr/lib/jvm/java-1.4.2-gcj-4.0-126.96.36.199 ant -Dbuild.compiler=gcj runDoclet
JAVA_HOME=/usr/lib/jvm/java-1.4.2-gcj-4.0-188.8.131.52 ant -Dbuild.compiler=gcj test-all
The first command creates the custom doclets, while the second command compiles the CDK and runs the JUnit tests. For Classpath developers: here's how to check out the cdk module from CVS.
The results are interesting: while Sun's JVM gives 11 problems, gij gives 399 problems. The test-all target creates a reports/result.txt document listing all failing tests, and I've put the diff -u for the two JVMs online. I will make diffs for jamvm, kaffe and cacao too.
I hope this gives the free Java community extra feedback on the excellent work they are doing.