activate new Gateway but use existing routers/extenders

I have a new Gateway I’m supposed to set up to replace the current xfinity modem we’ve had for a few years. We have a large sprawling house so have several Orbi routers/extenders as a network. All has worked great for many years. 

When I activate and plug in this new Gateway, I want it to use the current Orbi router network and its password etc. But I have a feeling that’s not going to work when I set up the new Gateway or the setup isn’t going to tell me how to do that. I’m hesitant to activate the Gateway because of this since we currently have no problems. Any suggestions or advice? (I’m pretty tech ignorant older guy.)