This work studied the manipulation of ferromagnetic behavior in gallium manganese nitride (GaMnN) epilayers and heterostructures. Key findings include: 1) GaMnN/p-GaN heterostructures exhibited room temperature ferromagnetism that increased with p-type GaN thickness and hole concentration. 2) GaMnN i-p-n structures showed electric field controlled ferromagnetism at room temperature, with magnetization decreasing under reverse bias due to p-GaN layer depletion. 3) Ferromagnetism in the structures depended on p-GaN layer thickness and GaMnN thickness, demonstrating manipulation of room temperature ferromagnetic properties.