I had a friendly email conversation with Daz DeBoer, who fixed the related bug
GRADLE-2076. The most relevant part was this:
Our current behaviour is just the ivy default behaviour, with the fix for handling types 'maven-plugin' and 'eclipse-plugin'. You're certainly right about needing a more general solution: probably just map "known" package types to their extensions, and everything else to 'jar'. We want to work the same as Maven when dealing with pom files.
A pull-request with the fix is probably the best way to get this bug prioritised .
I sympathize with the desire for a pull-request, but I'm just not familiar enough with the gradle codebase to even get started.