if you set up two or more ActiveGates of the same priority (so 2+ Environment AGs or 2+ Cluster AGs if there are no reachable Environment AGs for your Environment) that are both reachable by your OneAgents they will randomly connect to any one of them every few minutes which achieves active-active load balancing between the ActiveGates.
Also see this older question about the same topic (Security Gateway = ActiveGate): https://answers.dynatrace.com/spaces/482/dynatrace...
Any ActiveGates that your OneAgents can see will be in a 'group' that can be used by them. The agents constantly get updates as to what ActiveGates are available and will load balance among them and if any go offline or become unavailable in any way the agents will automatically switch to one of the working routes if there is one available. If you wanted to say have a 'group' of ActiveGates dedicated to a single environment you could download and install multiple Environment ActiveGates but there is no explicit ActiveGate Group like there was a Collector Group in AppMon as the ActiveGates aren't involved in the instrumentation of the monitored applications like in AppMon so which ActiveGate they send data through does not really matter.