shp2pgsql-gui: 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/shp2pgsql-gui - from package: postgis

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

On Debian possible default full paths are:

/usr/bin/shp2pgsql-gui - from package: postgis

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

Error in other languages:
shp2pgsql-gui: bevel nie gevind nie
shp2pgsql-gui: comandă negăsită
shp2pgsql-gui: comando não encontrado
shp2pgsql-gui: commande introuvable
shp2pgsql-gui: command not found
shp2pgsql-gui: käsku ei ole
shp2pgsql-gui: không tìm thấy lệnh
shp2pgsql-gui: komanda nerasta
shp2pgsql-gui: Kommando nicht gefunden.
shp2pgsql-gui: kommandot finns inte
shp2pgsql-gui: komut yok
shp2pgsql-gui: nie znaleziono polecenia
shp2pgsql-gui: níor aimsíodh an t-ordú
shp2pgsql-gui: no se encontró la orden
shp2pgsql-gui: no s'ha trobat l'ordre
shp2pgsql-gui: opdracht niet gevonden
shp2pgsql-gui: parancs nem található
shp2pgsql-gui: perintah tidak ditemukan
shp2pgsql-gui: príkaz nenájdený
shp2pgsql-gui: příkaz nenalezen
shp2pgsql-gui: команда не найдена
shp2pgsql-gui: командата не е открита
shp2pgsql-gui: 命令找不到
shp2pgsql-gui: コマンドが見つかりません