If you just want more than one dependency whenever someone references libs.jdom, then you don’t need client modules.
You can simply do:
ext.libs = [
jdom : [
'org.jdom:jdom:1.0',
'org.jdom:jaxen-jdom:1.0-FCS',
'org.jdom:jaxen-core:1.0-FCS'
]
}
]
It’s actually quite a useful mechanism: Optional means: “I also integrate with this other library if you use it. If not, that’s okay”.