gapcmon: command not found

Have You tried accessing the binary with the full path?

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/gapcmon - from package: apcupsd-gui

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