Assuming you only have one router you’d probably be fine with ff02::2 (that’s the well-known-multicast for every IPv6 router on the link).
Otherwise your ISP or vendor should have the default value in some kind of setup instructions I’d guess.
Edit: Oh, and then there is the Neighbour Discovery Protocol, a router advertises itself on the link. It can even give you a valid configuration through it. If not, you can at least look up the advertised address via a simple command line command.
Yes, if you get a standard gateway defined, either through a DHCP server (pretty much all consumer router should have one running by default too) or SLAAC, you should be able to look it up through the settings in the GUI, too.
You’re welcome. I’m no big expert myself, but I’d say that most things aren’t so different from v4, once you understand how v6 addresses work and are configured (and that there is no broadcast address anymore).