  We present the results of a direct-imaging survey for very large separation
($>$100 au), companions around 95 nearby young K5-L5 stars and brown dwarfs.
They are high-likelihood candidates or confirmed members of the young
($\lessapprox$150 Myr) $\beta$ Pictoris and AB Doradus moving groups (ABDMG)
and the TW Hya, Tucana-Horologium, Columba, Carina, and Argus associations.
Images in $i'$ and $z'$ filters were obtained with the Gemini Multi-Object
Spectrograph (GMOS) on Gemini South to search for companions down to an
apparent magnitude of $z'\sim$22-24 at separations $\gtrapprox$20" from the
targets and in the remainder of the wide 5.5' $\times$ 5.5' GMOS field of view.
This allowed us to probe the most distant region where planetary-mass
companions could be gravitationally bound to the targets. This region was left
largely unstudied by past high-contrast imaging surveys, which probed much
closer-in separations. This survey led to the discovery of a planetary-mass
(9-13 $\,M_{\rm{Jup}}$) companion at 2000 au from the M3V star GU Psc, a highly
probable member of ABDMG. No other substellar companions were identified. These
results allowed us to constrain the frequency of distant planetary-mass
companions (5-13 $\,M_{\rm{Jup}}$) to 0.84$_{-0.66}^{+6.73}$% (95% confidence)
at semimajor axes between 500 and 5000 au around young K5-L5 stars and brown
dwarfs. This is consistent with other studies suggesting that gravitationally
bound planetary-mass companions at wide separations from low-mass stars are
relatively rare.
