J.A.C.K. (previously JackHammer) does not know what is a gamepack.
You must download the fgd file yourself and configure J.A.C.K to use it.
You can follow this Tremulous tutorial and adapt it for Unvanquished.
Do nothing with .dpk files as their content is not supported. Copy files from the UnvanquishedAssets repository instead.
You're exploring an unknown land.