/usr/share/fish/completions/gpasswd.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | complete -c gpasswd -xa '(__fish_complete_groups)'
complete -c gpasswd -s a -l add -d 'Add user to group' -xa '(__fish_complete_users)'
complete -c gpasswd -s d -l delete -d 'Remove user from group' -xa '(__fish_complete_users)'
complete -c gpasswd -s h -l help -d 'Print help'
complete -c gpasswd -s r -l remove-password -d 'Remove the GROUP\'s password'
complete -c gpasswd -s R -l restrict -d 'Restrict access to GROUP to its members'
complete -c gpasswd -s M -l members -d 'Set the list of members of GROUP' -xa '(__fish_complete_list , __fish_complete_users)'
complete -c gpasswd -s A -l administrators -d 'set the list of administrators for GROUP' -xa '(__fish_complete_list , __fish_complete_users)'
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