“Institutional or disciplinary gatekeeping”

This means that the filtering happens through either:

Institutional mechanisms

Such as:

• funding committees;
• hiring committees;
• journals;
• universities;
• promotion systems.

or:

Disciplinary mechanisms

Such as:

• methodological norms;
• accepted standards of evidence;
• dominant theoretical frameworks;
• professional expectations.

For example:

A paper is rejected because reviewers say its methodology violates accepted historical standards.

That is potentially disciplinary gatekeeping.

If:

a university refuses to fund the project,

that is more clearly institutional gatekeeping.

They can overlap.