The Point-1 heading sensor will work just fine with the Gen-1 HDS units. I used it with my Gen-1 units for quite some time before I upgraded to newer units. Below is what Lowrance has to say about it.
Will Point-1 work with my HDS Gen1?
The Point-1 antenna/heading sensor is a standard NMEA2000® device. Therefore, it will connect to any NMEA2000® backbone and communicate with any NMEA device on that same network. The HDS Gen2 models, both keypad and touch, with software version 2.0 or higher will have GPS damping and filtering features, as well as the ability to initiate heading calibration from the HDS, that will not be present in any HDS Gen1.