Here you will find some basic queries and their setup.

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Release Pipeline Gate Check Work Item Query - Development Task Completion Validation

Browse to Boards > Queries and click "New Query"

Setup a Work Item Query as below that targets the feature that you're building the gate check for, the ID being the Feature ID. Select the Type of Query to be 'Tree of work items'.
Run the query to validate the result.

Click Save, save the query in the "Development Task Completion" folder

Also ensure that the folder has a security access profile for the "Project Build Service (Organisation)", otherwise the pipeline gate will fail in the process

Release Pipeline Gate Check Work Item Query - Test Case Validation

Browse to Boards > Queries and click "New Query"

Setup a Work Item Query as below that targets the feature that you're building the gate check for, the ID being the Feature ID. Select the Type of Query to be "Work items and direct links".
Run the query to validate the result.

Click Save, save the query in the "Test Cases" folder

Also ensure that the query folder has a security access profile for the "Project Build Service (Organisation)", otherwise the pipeline gate will fail in the process