Like other Bitcoin hard-forks, BitMEX will not support Bitcoin Gold (BTG).
At block 491407, the BTG team intends to snapshot balances. Note that the BTG client is not completed and will not be released at this time. Therefore, there is little risk of leaks through transaction replay, assuming the BTG team properly implements replay protection by the actual client/network launch.
BitMEX will process an additional round of withdrawals at 22:00 UTC (in 2 hours 40min) for users who wish to hold Bitcoin in their own address for the snapshot. It is safe to re-deposit after block 491407.