Proper noun
ぐんまけん
群馬県の実家から帰ってきたら、知らせてくれよな。
Let me know when you come back from your parents’ house in Gunma prefecture.
The flock of horses prefecture is the part of Japan known as Gunma Prefecture.
The reading is a bit weird. 群 is okay, but 馬 is just ま, which is an exception. It's like the ま of うま. Then there's prefecture which is the same too.