g_do_dssp: 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

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/g_do_dssp - from package: gromacs

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