First find the driver (something like /usr/lib/x86_64-linux-gnu/libmysqlclient.so.21).

It's name and location change depending on Linux versions, so Gestinux can't find it.

 sudo find / libmysql*.* | grep libmysql

if not found, try to install one, and retry the find :

 sudo apt-get install libmysqlclient*

Note the file found. It is possible to register it when you will create or select your Database using Gestinux.

You can also run the command line below, replacing the 2nd file by the driver path found before.

 sudo ln -s /usr/lib/x86_64-linux-gnu/libmysqlclient.so.21  /usr/lib/mysqlclient.so

After that go back to proceed with Gestinux installation

Read also

Install MariaDb or MySql client driver on Linux