Here is a video I took which shows some ArmA 3 gameplay with an FPS and GPU usage monitor at the top of the screen. (gpu usage is the one with % next to it)
https://www.youtube.com/watch?v=UkHSciKoSD4
The GPU usage is never at 100%.

Here are images of my CPU and GPU usage while playing:
http://cdn.overclock.net/c/cd/cd2e6603_ArmAIO.png
http://cdn.overclock.net/5/5d/5dfa99b6_Untitled.png

My system specs:
Core i7 2600k at 4.4Ghz
GTX 480
G1 Sniper 3 motherboard
1600MHz CL9 RAM
Western Digital Caviar Black 1TB hard drive
Corsair TX650V2 power supply