  The total gas mass of a protoplanetary disk is a fundamental, but poorly
determined, quantity. A new technique \citep{bergin+13} has been demonstrated
to assess directly the bulk molecular gas reservoir of molecular hydrogen using
the HD J=1-0 line at 112 $\mu$m. In this work we present a {\it Herschel} Space
Observatory\footnote{Herschel is an ESA space observatory with science
instruments provided by European-led Principal Investigator consortia and with
important participation from NASA.} survey of six additional T Tauri disks in
the HD line. Line emission is detected at $>$3$\sigma$ significance in two
cases: DM Tau and GM Aur. For the other four disks, we establish upper limits
to the line flux. Using detailed disk structure and ray tracing models, we
calculate the temperature structure and dust mass from modeling the observed
spectral energy distributions, and include the effect of UV gas heating to
determine the amount of gas required to fit the HD line. The range of gas
masses are 1.0-4.7$\times10^{-2}$ for DM Tau and 2.5-20.4$\times10^{-2}$ for GM
Aur. These values are larger than those found using CO for GM Aur, while the
CO-derived gas mass for DM Tau is consistent with the lower end of our mass
range. This suggests a CO chemical depletion from the gas phase of up to a
factor of five for DM Tau and up to two orders of magnitude for GM Aur. We
discuss how future analysis can narrow the mass ranges further.
