GPI triggers are physical connections between two pieces of equipment. Typically GPI triggers are based on a change of state on a monitored pin of the serial port. For instance you could create a GPI trigger which was operated by the alarm closure of an alarm system, or a port on a piece of broadcast equipment.
Because of the variety of configurations possible, your Stream system does not come with a GPI cable. Each GPI cable should be custom manufactured by someone who understands the risk and has adequate knowledge of both pieces of equipment to be connected. If you are not comfortable reading electrical schematics, and building custom cables you should provide the included schematic to a qualified technical engineer. (see Appendix A, GPI Schematic). Stream supports GPI inputs which can be configured in a variety of ways. Both COM ports are supported and the state of the trigger pins can be configured to match most situations.