How do I rename the Project Workflow stage gate names ?
Probably you mean statuses. The simplest and most intuitive way is to go to workflow editing, diagram mode and double click on the Status. Please check: https://confluence.atlassian.com/adminjiraserver072/working-with-workflows-828787890.html
Or just locate Statuses under Administration -> Issues.
If by "stage gate" you really mean "transition", then it's the same answer - go into the workflow, edit it, find the transition and rename it. See the doc Volodymyr already pointed you to.
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