Hello. Thank you for advise.
I was not aware of special documentation "R80.20.30 Centrally Managed Administration Guide". I have red the basic documentation and Smart-1 cloud documentation, nowhere was mentioned anything like that. Everywhere was just mentioned that in management cluster object needs to be pre-created with dummy GW first. The conversion of standalone GW to cluster is not supported.
In my case all nodes has own IP on each interface and own virtual IP for each clustered interface. (included WAN - Public IP) + dedicated sync interface (set under topology)
As you do change all settings on active node only, as the standby will sync the changes automatically, you usually only use the virtual cluster IP !
I suppose this is not true for the OS basics such a DHCP, don't you?