GMSL1 parts takes parallel data in at PCLK and output at the other end with a regenerated PCLK. It doesn't matter what the data is, it is just sampled at PCLK and so must be slower than that. We have customers using these parts as non-video SerDes.
The video related pins like HS can be treated as data unless you program the device to do something with them, which you would only do for video. All the input pins are independent, so you could use them for GPIO input to the serializer and output from the deserializer. In general (I mean most parts in this series) there is one GPI on the deserializer which is output at the serializer. Some parts have additional GPIO that is written by I2C. To get more GPIO, you can add an I2C controlled IO expander.
The simplest parts are MAX96705/6 To get the most user pins and GPIO look at the MAX9275/6. All the parts in this series interwork.