the public managed security gateway is not limited to specific tenants - you do not need to specify the list of tenants as for the private gateway. In other words - it has always been multi-tenant.
well, could you describe the use case - what do you want to accomplish? E.g. if private gateway functionality is needed, why not install the private gateway instead?
Just to clarify - both gateways from the binary perspective are the same, what is different is the configuration. But configuration differences are quite important and you cannot easily convert one to another.
Public managed gateway by definition does not have tenant restrictions (the tenant must be present on the cluster of course). Main purpose of public managed gateway is to handle agentless RUM, mobile and synthetic - "beacon traffic". It can be used of course also to connect one agents, usually the assumption is the agents would connect from outside.
Private gateway on the other hand - imposes the restrictions on the tenants - by default single one but can be a list as well. It is usually running in private network and cannot handle the "beacon traffic."
hope this helps,