Please help me on this below questions.
Is the ESM a separate product which must be installed and licensed?
Is the ESM already part of the current installation?
ESM is a pre-installed component of gateway, which is provided along with gateway. You just need to enable it for monitoring. But it requires individual license.
Below is how you configure and enable it:
Configure the Enterprise Service Manager - CA API Gateway - 9.0 - CA Technologies Documentation
I hope this helps.