If you're looking for manga similar to Girls und Panzer, you might like these titles. All recommendations are made by Anime-Planet users like you!
Both series are based on military vessels from WW II era, Aoki features Warships from the period whereas GuP features Tanks from same era.
Both series are essentially Girls operating the Vessels/Tanks. While GuP is all girls and is light hearted competetion, Aoki is world of adults with massacares and dark themed. All in all both are excellent military series and provide ample knowledge about vehicles of WWII era and their power.