gpsk31: 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/gpsk31 - from package: gpsk31

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

Error in other languages:
gpsk31: bevel nie gevind nie
gpsk31: comandă negăsită
gpsk31: comando não encontrado
gpsk31: commande introuvable
gpsk31: command not found
gpsk31: käsku ei ole
gpsk31: không tìm thấy lệnh
gpsk31: komanda nerasta
gpsk31: Kommando nicht gefunden.
gpsk31: kommandot finns inte
gpsk31: komut yok
gpsk31: nie znaleziono polecenia
gpsk31: níor aimsíodh an t-ordú
gpsk31: no se encontró la orden
gpsk31: no s'ha trobat l'ordre
gpsk31: opdracht niet gevonden
gpsk31: parancs nem található
gpsk31: perintah tidak ditemukan
gpsk31: príkaz nenájdený
gpsk31: příkaz nenalezen
gpsk31: команда не найдена
gpsk31: командата не е открита
gpsk31: 命令找不到
gpsk31: コマンドが見つかりません