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.
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.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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