`git gone`

git-gone

#!/bin/bash

usage() {
  cat <<EOF
usage: git gone [-pldD] [<branch>=origin]
OPTIONS
        -p      prune remote branch
        -n      dry run: list the gone branches
        -d      delete the gone branches
        -D      delete the gone branches forcefully
EXAMPLES
git gone -pn    prune and dry run
git gone -d     delete the gone branches
EOF
}

POSITIONAL=()
OPTIONS=()

while [[ $# -gt 0 ]]
do
  key="$1"

  case $key in
    -*)
    OPTION="$1"
    shift # past argument
    ;;
    *)    # unknown option
    POSITIONAL+=("$1") # save it in an array for later
    shift # past argument
    ;;
  esac
done
set -- "${POSITIONAL[@]}" # restore positional parameters

if [ $# -eq 0 ]; then
  BRANCH="origin"
elif [ $# -eq 1 ]; then
  BRANCH="$1"
else
  usage && echo "too many arguments!" && exit 1
fi

(( prune=0 ))
(( list=0 ))
(( delete=0 ))
(( force=0 ))
for (( i=1; i < ${#OPTION}; i++ )); do
  op="${OPTION:$i:1}"
  case $op in
    p)
    (( prune=1 ))
    ;;
    n)
    (( list=1 ))
    ;;
    d)
    (( delete=1 ))
    ;;
    D)
    (( delete=1 ))
    (( force=1 ))
    ;;
    *)
    usage && echo "unknown option $op!" && exit 1
    ;;
  esac
  OPTIONS+=("$op")
done

if [ ${#OPTION} -eq 0 ]; then
  usage && exit 1
fi
if [ $prune -eq 1 ]; then
  git remote prune "$BRANCH"
fi

gone="$BRANCH/.*: gone]"
if [ $delete -eq 1 ]; then
  if [ $force -eq 1 ]; then
    git branch -vv | grep "$gone" | awk '{print $1}' | xargs git branch -D
  else
    git branch -vv | grep "$gone" | awk '{print $1}' | xargs git branch -d
  fi
elif [ $list -eq 1 ]; then
  git branch -vv | grep "$gone"
fi

I have no memory of where I found this, but it's super handy.
Stick it somewhere in your path and git will find it when you run git gone.
Run it without arguments to see how to use it.