Hello Mariusz
DHCPv6 does not typically set the default gateway for IPv6 clients. The default gateway is typically distributed through Router Advertisements (RA) sent by routers, not through DHCPv6. Indeed, DHCPv6 has no standard option specifically designed for directly assigning the default gateway to clients in the same way that it does for IPv4. This is a significant difference from IPv4, where the default gateway is often distributed via DHCP.
You must configure the default gateway on the clients based on the Router Advertisement messages from the local router, which will include the default gateway information.
I hope this has been helpful!
Laz