  Advances in instrumentation have recently extended detailed measurements of
gas kinematics to large samples of high-redshift galaxies. Relative to most
nearby, thin disk galaxies, in which gas rotation accurately traces the
gravitational potential, the interstellar medium (ISM) of z>1 galaxies is
typically more dynamic and exhibits elevated turbulence. If not properly
modeled, these effects can strongly bias dynamical mass measurements. We use
high-resolution FIRE-2 cosmological zoom-in simulations to analyze the physical
effects that must be considered to correctly infer dynamical masses from gas
kinematics. Our analysis covers a range of galaxy properties from low-redshift
Milky-Way-mass galaxies to massive high-redshift galaxies (M_* > 10^11 M_sun at
z=1). Selecting only snapshots where a disk is present, we calculate the
rotational profile v_phi(r) of the cool (10^3.5 K < T < 10^4.5 K) gas and
compare it to the circular velocity v_c=sqrt(GM/r). In the simulated galaxies,
the gas rotation traces the circular velocity at intermediate radii, but the
two quantities diverge significantly in the center and in the outer disk. Our
simulations appear to over-predict observed rotational velocities in the
centers of massive galaxies (likely from a lack of black hole feedback), so we
focus on larger radii. Gradients in the turbulent pressure at these radii can
provide additional radial support and bias dynamical mass measurements low by
up to 40%. In both the interior and exterior, the gas' motion can be
significantly non-circular due to e.g. bars, satellites, and inflows/outflows.
We discuss the accuracy of commonly-used analytic models for pressure gradients
(or "asymmetric drift") in the ISM of high-redshift galaxies.
