First of all you should consider getting the 3rd party library as OSGi bundle and add it as required or import the packages. Have a look at Springsource Enterprise bundle repository (http://www.springsource.com/repository/app/) or eclipse orbit (http://www.eclipse.org/orbit/).

If you still want to embed the 3rd party library you need to add the Bundle-ClassPath header to your manifest:

instruction ‘Bundle-ClassPath’, ‘third-party.jar’

But this will only tell OSGi to consult the bundle classpath when looking for classes. You still have to tell gradle that you want to embed the 3rd party jar. I think it should be possible to to this through configuring the jar task – but I haven’t done that myself. If that won’t work you still could add the 3rd party jar as resource – which feels a bit ‘hacky’:

sourceSets {
main {
scala {
srcDir ‘src/main/scala’
}
resources {
srcDir ‘lib’
}
}
}

Assuming your 3rd party lib resides in the ‘lib’ folder.
I hope this was helpful.

/Mike