  The 6H-perovskites Ba$_3$RRu$_2$O$_9$ (R = rare earth element) demonstrate
the magnetodielectric (MD) coupling as a manifestation of $4d - 4f$ magnetic
interactions. Here, we have reported a detailed study of the structural,
magnetic, heat capacity, and MD properties of the 6H-perovskite
Ba$_3$GdRu$_2$O$_9$. The signature of long-range antiferromagnetic (AFM)
ordering $\sim$ 14.8 K ($T_N$) is evident from the magnetization and heat
capacity studies. The $T_N$ shifts towards the lower temperature side, apart
from splitting in two with the application of the magnetic field.
Field-dependent magnetization at 1.8 K shows three metamagnetic transitions
with the opening of small hysteresis in different regions. A new transition at
$T_1$ emerges after the onset of the first metamagnetic transition. Complex
magnetic behavior is observed in different magnetic field regions whereas these
field regions themselves vary with the temperature. Dielectric response
recorded at zero and 80 kOe field exhibits the development of MD coupling well
above $T_N$. The MD coupling ($\sim$ 4.5 \% at 10 K) is enhanced by 25 \% as
compared to the Dy counterpart. Effect of complex magnetic behavior is also
conveyed in the MD results where the maximum value of MD coupling is observed
in the vicinity of 10 K (onset of $T_1$) and near the second metamagnetic
transition. Our investigation suggests that both Gd and Ru moments align
simultaneously at $T_N$. Short-range magnetic correlations are possibly
responsible for MD coupling above $T_N$.
