If you have a 200A main panel with a 100A feeder to the Gateway, then you have to meet one of these two limits: (a) the other breakers (excluding the main breaker) in your main panel have to add up to less than 100A (per pole, so you could have (10) 20A single pole breakers, as long as they are evenly distributed among the two buses) or (b) your inverters behind the Gateway have to add up to less than 7.7 kW. For (b), solar inverters definitely count, and each Powerwall counts as 5.8 kW, unless you are under the 2020 NEC, or your AHJ reasonably allows the use of the 2020 NEC provisions on Power Control Systems, even if they haven't adopted it officially yet.
Cheers, Wayne