IPv6 uses NDP to find out information about default gateway where in case of IPv4, the default router/gateway can be either configured or can be receive through DHCP.
NDP uses ICMPv6 messages:
Router Solicitation (ICMPv6 Type =133): Hosts send NS message to all the routers on the network using a multicast address (FF02::2). Hosts can learn about all the routers in the subnet.
Routers on the network respond using Router Advertisement (ICMPv6 Type =134) message. They can send this information directly to the host who initiated the request or they can send this information to all IPv6 nodes on the network using IPv6 Multicast address FF02::1. RA message contains link-local IPv6 of router (default-gateway)
In this way, host machine can have the default gateway information.