This function selects a gate source for a subdevice. The gate_index parameter selects which gate is being configured, should the subdevice have multiple gates. It takes a value from 0 to N-1 for a subdevice with N different gates. The gate_source parameter selects which signal you wish to use as the gate, and is also driver-dependent. If the subdevice does not support configuring its gates on a per-channel basis, then the channel parameter will be ignored.
You may query the gate source with the comedi_get_gate_source function. This function is only useable on subdevices that provide support for the INSN_CONFIG_SET_GATE_SRC configuration instruction.