The error subcommand given by git tab completion.

On Ubuntu 20.04, the `git TAB' will give me the following available subcommands:

$ git TAB add deborig merge rm am describe mergetool send-email apply diff mv shortlog archive difftool notes show bisect fetch pull show-branch blame format-patch push sparse-checkout branch fsck range-diff stage bundle gc rebase stash checkout gitk reflog status cherry grep remote submodule cherry-pick gui repack switch citool help replace tag clean init request-pull whatchanged clone instaweb reset worktree commit latexdiff restore config log revert

Then I try the following:

$ git gui git: 'gui' is not a git command. See 'git --help'.

The most similar commands are
    gc
    grep
    init
    pull
    push

The git version is shown as below:

$ git --version git version 2.25.1

As you can see, gui is among the auto-completed subcommands list, but it can be run like the manner of other subcommands. Any hints for this problem?

Regards