gmail-imap-label: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/gmail-imap-label - from package: libnet-gmail-imap-label-perl

You may install the required package with command apt-get install {package}

Error in other languages:
gmail-imap-label: bevel nie gevind nie
gmail-imap-label: comandă negăsită
gmail-imap-label: comando não encontrado
gmail-imap-label: commande introuvable
gmail-imap-label: command not found
gmail-imap-label: käsku ei ole
gmail-imap-label: không tìm thấy lệnh
gmail-imap-label: komanda nerasta
gmail-imap-label: Kommando nicht gefunden.
gmail-imap-label: kommandot finns inte
gmail-imap-label: komut yok
gmail-imap-label: nie znaleziono polecenia
gmail-imap-label: níor aimsíodh an t-ordú
gmail-imap-label: no se encontró la orden
gmail-imap-label: no s'ha trobat l'ordre
gmail-imap-label: opdracht niet gevonden
gmail-imap-label: parancs nem található
gmail-imap-label: perintah tidak ditemukan
gmail-imap-label: príkaz nenájdený
gmail-imap-label: příkaz nenalezen
gmail-imap-label: команда не найдена
gmail-imap-label: командата не е открита
gmail-imap-label: 命令找不到
gmail-imap-label: コマンドが見つかりません