If I got a new Windows server for testing and I want to create all of the products in API Management - I know Policy Manager as well as MAG/OTK/SDK to that can all be installed to Windows server, but the API Gateway needs a RHEL/CentOS/Linux server correct? But API Portal and Live API Creator can be done through a web browser?
You are correct, the API Gateway software does run on Linux (RHEL/CentOS). The policy manager (GUI interface to the Gateway software) can run on Windows, Linux and Mac OS. Additionally, there is also a Web Interface for the policy manager.
Using the Policy Manager Browser Client - CA API Gateway - 9.3 - CA Technologies Documentation
Requirements and Compatibility - CA API Gateway - 9.3 - CA Technologies Documentation
MAG & OTK are installed as Solution Kits through the policy manager interface.
API Portal (v 3.5 & 4.x) runs on RHEL, but additionally 4.2 makes uses of docker containers (on RHEL/CentOS). The interface to using these products is a web browser.
API Portal Prerequisites - CA API Developer Portal - 4.2 - CA Technologies Documentation
As for Live API Creator the requirements can be found below. Similar to portal in the sense that the software should be running on a support platform/ application server and the GUI will be accessed via a Web Browser.
Installation Requirements and Supported Platforms - CA Live API Creator - 4.1 - CA Technologies Documentation
Hope this helps.