GMSL1 has Data Rate Select (DRS) pins (or bit) to support the low PCLK from camera (less than 12.5MHz).
When setting the DRS pin to high, the serializers will replicate every pixel and send them over the link. The data rate will be double. The purpose of this pixel replication is to comply the minimum required PCLK frequency on the link (12.5MHz).
At the deserializers, if the DRS pin is set to high, the replicated pixels will be deleted and the original pixels will be sent to the output. If the DRS is set to low, the deserializers keep both original and replicated pixels and send them to the output. As a result, the output data rate is double the input data rate.
In summary, users should keep DRS pins setting at serializer and deserializer the same to have the same data rate of input and output.