It seems the Java library doesn't have this feature (yet).
You can set this info manually, though:
GSFileOptionsBuilder optionsBuilder = new GSFileOptionsBuilder()
.setBucket("my_bucket")
.setKey("my_object")
.setAcl("public-read")
.setMimeType("text/html") //etc etc
.setUserMetadata("date-created", "01/09/2011", "owner", "Jon");
And retrieve this piece of metadata with
GcsFileMetadata.getOptions().getUserMetadata().get("date-created")