If the 3CX outbound rules are correctly routing each call to the correct trunk, but, the GSM gateway is not honouring that selection, and continues to send the calls out on the first SIM, then you have not correctly split the gateway into 4 individual trunks. Not being familiar with the model, i don't know how they accomplish that. It may be done using the port, or by routing calls, using a prefix, attached to the number. Incoming calls must also be directed to the correct trunk.
Review the gateway settings.