ICAO Annex 6 ‘Operation of Aircraft’, applies to all States (National Authorities).
6.15.1 All turbine-engined aeroplanes of a maximum certificated take-off mass in excess of 5 700 kg or authorized to carry more than nine passengers shall be equipped with a ground proximity warning system.
Various amendments to this and the documents below, upgrade the requirement to require a predictive function i.e. EGPWS / TAWS.
JAA (EASA) JAR-OPS 1, Regulation – Applies to all Operators.
1.665 Ground proximity warning system and terrain awareness warning system.
(a) An operator shall not operate a turbine powered aeroplane having a maximum certificated take-off mass in excess of 5 700 kg or a maximum approved passenger seating configuration of more than 9 unless it is equipped with a ground proximity warning system.
FAA, FAR 121, 91, and 135, also AC25-23. Similar to JAR, but there are some significant dispensations e.g. cargo/freight operations excluded !!!!
121.354 …. no person may operate a turbine-powered airplane unless that airplane is equipped with an approved terrain awareness and warning system, including a terrain awareness display, that meets the requirements for Class A equipment of TSO-C151a.