icewm-set-gnomewm: 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 CentOS possible default full paths are:

/usr/bin/icewm-set-gnomewm - from package: icewm

You may install the required package with command yum install {package}

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