how to prevent an Gated Check-In to put the Keyword ***NO_CI*** in the Comment

You can disable ***NO_CI*** comment by setting the NoCIOption property of the SyncWorkspace workflow activity to false. A similar question and answer can be found here.

You can either follow this blog post to implement a parameter for your build definition, or you can directly set this property to 'false' in your Gated CheckIn build template.

vNext Builds - preventing the ***NO_CI***

For vNext builds, check the "Run continuous integration triggers for committed changes" option on the Triggers tab for a build.

---

---

See the section "Option to run CI builds" in this reference:: https://docs.microsoft.com/en-us/vsts/build-release/concepts/definitions/build/triggers#gated