you've guessed it right.
You can use that field to provide a different ssh key that will be used for that specific config.
You can put a dss group in the Group field, so DSS will use this GIT config every time a member of that group is trying to perform a GIT action.
In the Configuration options you can provide this key-value to point DSS to the suitable key to use. DSS will indeed need to be able to reach and read the key
core.sshCommand -> /path/to/the/key
Hope that helps. Take care,
Architect @ Dataiku