The GCR was able to use natural uranium as fuel, enabling the countries that developed them to fabricate their own fuel without relying on other countries for supplies of enriched uranium, which was at the time of their development in the 1950s only available from the United States or the Soviet Union.
The Canadian CANDU reactor, using heavy water as a moderator, was designed with the same goal of using natural uranium fuel for similar reasons.
Both types were mainly constructed in their countries of origin, with a few export sales: two Magnox plants to Italy and Japan, and one UNGG to Spain.
More recently, GCRs based on the declassified drawings of the early Magnox reactors have been constructed by North Korea at the Yongbyon Nuclear Scientific Research Center.
Both types were, in their countries of origin, also designed and used to produce weapons-grade plutonium, but at the cost of major interruption to their use for power generation despite the provision of online refuelling.