Szia Laszlo!
I read the Java policy, but I has not became more wise. Say, I have a complex application, which licensed under GPL v2. But it does not compile with free tools. The JVM is not a problem, as Debian has j2sdk1.4
No it doesn't, and it cannot for licensing reasons.You can still depend on j2sdk1.4 since that package is provided by sources outside of Debian. You should add '| sun-j2sdk1.4', which is the package automatically generated by j2se-package. (should we have this kind of instructions in the policy?)
What free tools did you try that did not work? Did you report bugs? , but what's if the application needs SUN's JavaMail as well?
What should I write to debian/control's Build-Depends field? libgnumail-java is not enough, as that misses some classes the application needs.
Then if there is no debian package (even outside Debian) providing an implementation of javamail that works for your package, I don't think you can write a proper Build-Depends.
How many classes does libgnumail-java miss? It might be quite easy to at least include stub classes that would allow your app to be compiled with free tools.
Daniel