/usr/lib/jvm/java-1.5.0-gcj-6-amd64/src.zip is in gcj-6-source 6.3.0-18+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/jvm/java-1.5.0-gcj-6-amd64/../../../share/java/libgcj-src-6.zip
/usr/lib/jvm/java-1.5.0-gcj-6-amd64/../../../share/java/libgcj-src-6.zip