Can someone please let me know which product is better in between : ESM and GMU and what are the advantages of one product over the other.
Which is better is a matter of personal preference and use case. ESM provides some functionality that is not available in the GMU. The below links will provide a good start in getting a idea of each products capabilities.
Gateway Migration - CA API Gateway - 9.2 - CA Technologies Documentation
CA API Gateway 8.3.00 released
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