Custom Fields can be displayed but not updated. Not sure if this is already implemented in the gem.

netbox_identifier = 999
netbox_record= NetboxClientRuby.virtualization.virtual_machines.find_by(id: netbox_identifier)
puts netbox_record.custom_fields
netbox_record.custom_fields['my_field'] = "New Value"
netbox_record.save

completes without any error, but does not update the custom fields in netbox. Updating the name or description however works perfectly.