R80.40 JHF 48
(Host "moon" is the SmartCenter; "earth" is the gateway)
 
[Expert@moon:0]#  mgmt_cli -u admin --format json set simple-gateway name "earth" color "gold
Password: 
{
  "color" : "black",
  "icon" : "NetworkObjects/gateway",
...
...
  "name" : "earth",
  "sic-name" : "CN=earth,O=moon....",
  "anti-virus" : true
}
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Time: [21:10:35] 8/7/2020
---------------------------------------------
"Publish operation"  succeeded  (100%)  
 
 
So I guess this thing really doesn't work, eh?  I could make changes to host objects, but nothing works on gateway or a management host object.  API changelog shows "simple-gateway" isn't a new command either, ... 
This was really lovely during the gateway version upgrade when I needed to change the platform version to install policy and it didn't work as it was intended.  Thankfully, I knew how to backdoor an SSH port-forward tunnel from my remote management client to the SmartCenter... got in with SmartConsole just to click "OK" on a stupid dialog box and push policy and got it going.  Very lovely indeed... (insert "scowling face" emoji).
 
 Or am I holding it wrong again?
 
(now where's that "shrug" emoji... it was here somewhere...)