  Combining experimental investigations and first-principles DFT calculations,
we report physical and magnetic properties of Gd-substituted Y$_2$CoMnO$_6$
double perovskite, which are strongly influenced by antisite-disorder-driven
spin configurations. On Gd doping, Co and Mn ions are present in mixed-valence
(Co$^{3+}$, Co$^{2+}$, Mn$^{3+}$ and Mn$^{4+}$) states. Multiple magnetic
transitions have been observed: i) paramagnetic to ferromagnetic transition is
found to occur at \textit{T}$_C$=95.5 K, ii) antiferromagnetic transition at
\textit{T}$_N$=47 K is driven by $3d-4f$ polarisation and antisite disorder
present in the sample, iii) change in magnetization below \textit{T}$\leq$20 K,
primarily originating from Gd ordering, as revealed from our DFT calculations.
AC susceptibility measurement confirms the absence of any spin-glass or
cluster-glass phases in this material. A significantly large exchange bias
effect (\textit{H}$_{EB}$=1.07 kOe) is found to occur below 47 K due to
interfaces of FM and AFM clusters created by antisite-disorder.
