The following checks the value of ${module.gke-network.network_name}, if it's an empty string it assigns the network_name variable a value of "default" else if it's not an empty string it assigns the value of ${module.gke-network.network_name} to the network_name variable

network_name = "${module.gke-network.network_name} == "" ? "default" : module.gke-network.network_name}"

This is quite handy as it's saying "if I have set a value for ${module.gke-network.network_name} then use that value, if not just use the string "default"