Assuming you are talking about the cpu fan header, it seems to only offer pwm control for current Z87 boards (and Z77 as well). Older models like Z68 indeed had both pwm and voltage control. Asus boards are the same, in fact they ditched voltage control earlier than Gigabyte.
I don't know which mobo you have but for instance on Z87-UD3H the casefanheaders are all voltage controlled. If you have a spare one you could use a casefanheader for the cpu fan and control speeds using Speedfan.