Gunma Prefecture
proper noun
The flock of horses prefecture is the part of Japan known as Gunma Prefecture.
ぐんまけん
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.
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.