wood gate

Hello,
My name is Jaime and i have wood fence gate project and i need some advice! the area that i will be working on is about 9 feet wide. I'm planning to put the gate door in the middle and I believe the average gate door is about 3 feet wide. The fence will be a board on board with a 2x4 cap, 1x4 trim, and 2x6 base. the gate door is where i believe i am having hard time because i would like the board on board with the cap, trim, and base. I fear it would sag. so i need some advice on on how i could build this door with out sagging, I guess what kind of posts I should use 6x6 or steel posts, please i need help, thanks