You can monitor it through the rest api on the UAG
https://<your device IP or name>:9443/rest/v1/monitor/stats
However, someone took it a step future and created a module for powershell to query it:
http://andrewmorgan.ie/2018/03/viewing-vmware-unified-access-gateway-statistics-with-rest/
If you have vROPs, it will monitor the UAG through the broker agent on the connection servers.
