The 3005 will accept IP addresses from a DHCP server to assign to clients, it ignores any other DHCP parameter it receives from the DHCP server.
To set the DNS servers for the VPN client, you define these under the group settings under the General tab.
The Default Gateway will always just be set to the assigned IP address as once the tunnel is established the PC internally sends everything to the VPN client IP which then forwards it over the tunnel. This is similar to if you dialup to an ISP using a PPP connection, you'll notice your DG is always the same as your negotiated IP address.
If you do want to send all your tunnelled traffic through to a specific internal router/gw, then you can have the 3005 do this by setting the "Tunnel Default Gateway" parameter under Config - System - IP Routing - Default Gateways. You won't see this parameter appear on the client, but the 3005 will send all traffic it receives from clients to this address automatically.