  One of first the stages of planet formation is the growth of small
planetesimals and their accumulation into large planetesimals and planetary
embryos. This early stage occurs much before the dispersal of most of the gas
from the protoplanetary disk. At this stage gas-planetesimal interactions play
a key role in the dynamical evolution of \emph{single} intermediate-mass
planetesimals ($m_{p}\sim10^{21}-10^{25}g$) \emph{through gas dynamical
friction} (GDF). A significant fraction of all Solar system planetesimals
(asteroids and Kuiper-belt objects) are known to be binary planetesimals (BPs).
Here, we explore the effects of GDF on the evolution of \emph{binary}
planetesimals embedded in a gaseous disk using an N-body code with a fiducial
external force accounting for GDF. We find that GDF can induce binary mergers
on timescales shorter than the disk lifetime for masses above
$m_{p}\gtrsim10^{22}g$ at 1AU, independent of the binary initial separation and
eccentricity. Such mergers can affect the structure of merger-formed
planetesimals, and the GDF-induced binary inspiral can play a role in the
evolution of the planetesimal disk. In addition, binaries on eccentric orbits
around the star may evolve in the supersonic regime, where the torque reverses
and the binary expands, which would enhance the cross section for planetesimal
encounters with the binary. Highly inclined binaries with small mass ratios,
evolve due to the combined effects of Kozai-Lidov cycles with GDF which lead to
chaotic evolution. Prograde binaries go through semi-regular Kozai-Lidov
evolution, while retrograde binaries frequently flip their inclination and
$\sim50\%$ of them are destroyed.
