RS-232 is a two way, point to point protocol. In that context it makes no sense to “broadcast” to devices in parallel. Each client assumes that it is the only client on the link. In very simple cases, where you will be sending the same command to all clients and you will ignore replies (if any) by not even connecting the send wire from the clients, you could possibly get away with a parallel connection to the client receive terminals.

I don’t recommend attempting this approach.

What kind of controller will you be using? You can’t simply trigger a GC-100 to send an RS-232 command.