Can't replace character with Backslash with gsub

Elastic Stack Logstash
2

As you have found, you cannot have a backslash as the last character in a quoted string. Using a character group is the normal trick for the pattern. If you use it in the replacement string it is a literal, not a group. So you could do

mutate {
    gsub => [
        "PoundedUNC", "#", "\]",
        "PoundedUNC", "]", ""
    ]
}

It does not have to be ], you can use any character that will never appear in the UNC.