Keep in mind, the Fusion then is all setup and wired for the Fusion interconnect method. You will need to get new cabling between the radio and remote, using N2K backbone, the existing wiring isn't compatible with it. Fusion sells the correct interconnect cabling kits to get between the N2K backbone and the devices. Specifically you will need to have the new drop cables, T's and terminators. The power backbone should have been competed with the GPS antenna. You will also need additional backbone cable between the remote and head unit. The starter kit is a good option. The worst part will be getting that new cable between the remote and head unit, as all that is done before they put the boat together. Boat manufactures don't seem to realize that people actually modify and add things after the sale, and because they don't leave any way to get new cabling between point A and point B its a major chore to do it.
The reason for the N2K network to the GPS is because, the GPS receiver module is N2K based and not the older NMEA0183 method. The GPS antenna is actually a fully self contained receiver that outputs N2K PGNs to the chart plotter. There are also proprietary PGNs as well that can be used for setup etc.
Like XIXP mentioned you can also interface the engines into the N2K network as well with the gateway module, and allow full engine monitoring on the plotter too. The generator can also be interfaced as well, and the plotter should be able to be setup for "three engines". I haven't worked with the larger Garmin plotters, so I am not 100% sure on that.
2012 - Regal 35 Sports Coupe - Twin VP 5.7GiCE-300-P - DP-SA
2011 - Sea-Doo RXT260s - 1503HO Rotax 4TEC
2007 - Regal 3060 - Twin VP 5.0OSi - XDP
2002 - Crownline 230BR VP 8.2GSiPEFS - DP-SM
2001 - Polaris Genesis I - 1200DI