The Lowrance fuel flow sensor is not at all accurate, compared to the information you will get from the ECM, if you can connect direct to that. I had one on my last boat, combined with the Lowrance dash unit. No matter how often you calibrated it, it was hard to get under 10% error. It was useful in my case, as the boat had a 150 l + underfloor tank with gunwhale filler and no fuel gauge. The error in flow was always on the high side, too, so you always had more fuel left than the totaliser told you. And, of course, you have to input any fuel manually when you fuel up. I used to drain the tank twice a year to keep accurate track of contents.
Reading what you have italicised there, why would you connect o the Merc gateway, if it was not reading something from the ECM? That Lowrance sender will network direct to your HDS via the NMEA 2000 system. So, if you are connecting to the motor, you would be reading from the ECM, and, hence, no need for the Lowrance fuel flow sensor. ECM's meter very accurately.