gif2apng: 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/gif2apng - from package: gif2apng

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

Error in other languages:
gif2apng: bevel nie gevind nie
gif2apng: comandă negăsită
gif2apng: comando não encontrado
gif2apng: commande introuvable
gif2apng: command not found
gif2apng: käsku ei ole
gif2apng: không tìm thấy lệnh
gif2apng: komanda nerasta
gif2apng: Kommando nicht gefunden.
gif2apng: kommandot finns inte
gif2apng: komut yok
gif2apng: nie znaleziono polecenia
gif2apng: níor aimsíodh an t-ordú
gif2apng: no se encontró la orden
gif2apng: no s'ha trobat l'ordre
gif2apng: opdracht niet gevonden
gif2apng: parancs nem található
gif2apng: perintah tidak ditemukan
gif2apng: príkaz nenájdený
gif2apng: příkaz nenalezen
gif2apng: команда не найдена
gif2apng: командата не е открита
gif2apng: 命令找不到
gif2apng: コマンドが見つかりません