I run ANTLR through my IDE, not via gradle - sorry, I know that's bad form!
So the settings have to be set up manually...
For Intellij we do this :
Tools -> Configure ANTLR ...
Output Director : <PROJECT_ROOT>/feather-core/src/main/java
Package uk.co.nickthecoder.feather.grammar
Language Java
Generate Parse Tree Listener YES
Generate Parse Tree Visitor NO
Then delete the generated stuff in feather-core/src/main/java/uk.co.nickthecoder.feather.parser
Then open FeatherParser and Ctrl+Shift+G to generate.