Army, Troops, Force
noun, suffix
The kanji and the word are exactly the same. That means they share meanings as well.
ぐん
This has the same reading as the kanji you learned, meaning you know the reading!
彼は軍に入ることを決心した。
He decided to join the military.
その国は軍を強化しているようだ。
That country seems to be strengthening its military.
どう、軍の生活は？トレーニングはやっぱり大変？
How's life in the military? Is training hard?