Turbulence regulates angular momentum transport and influences dust evolution in protoplanetary disks. Measuring its strength is challenging through gas line observations because it requires data at high spatial and spectral resolution, and needs an exquisite determination of the disk temperature as well. In this work, we investigate the gap contrast observed in high angular resolution continuum images as a probe for turbulence in detail. By analyzing radiative transfer models of disks with parameterized gaps, we find that the millimeter gap contrast increases with decreasing turbulence, but it is also affected by the gap depth in the underlying surface density, scale height, and optical depth of the disk. We further take the HD 163296 disk as an application to illustrate that a simultaneous fitting to the broadband spectral energy distribution, and the azimuthally-averaged surface brightness and surface brightness along the disk minor axis, is necessary to reduce the degeneracy. Our radiative transfer analysis suggests low turbulence levels of alpha ~ 3*10^-5 for the HD 163296 disk, which is consistent with literature results, and is also below the upper limit set by gas line observations.
