In GMSL1 link, GPO on the serializer (MAX9275) follows GPI transitions on the deserializer (MAX9276). This means users can use this GPI/GPO function to transmit signals from the deserializer to serializer.
If the users does not use I2S function on serializer, he can use the SD pin to transfer the signal from serializer to deserializer as a GPIO. Please make sure that the I2S function is disabled.