The petitioner was convicted in a Japanese court of smuggling arms into the country. He returned to the US, bought a gun, and was charged with being a felon in possession. The Supreme Court interpreted GCA 68's reference to "convicted in any court" to mean any US court. Majority opinion by Breyer notes:

Past foreign convictions for crimes punishable by more than one year’s imprisonment may include a conviction for conduct that domestic laws would permit, for example, for engaging in economic conduct that our society might encourage. See, e.g., Art. 153 of the Criminal Code of the Russian Soviet Federated Socialist Republic, in Soviet Criminal Law and Procedure 171 (H. Berman & J. Spindler transls. 2d ed. 1972) (criminalizing “Private Entrepreneurial Activity”); Art. 153, id., at 172 (criminalizing “Speculation,” which is defined as “the buying up and reselling of goods or any other articles for the purpose of making a profit”); cf. e.g., Gaceta Oficial de la Republica de Cuba, ch. II, Art. 103, p. 68 (Dec. 30, 1987) (forbidding propaganda that incites against the social order, international solidarity, or the Communist State). They would include a conviction from a legal system that is inconsistent with an American understanding of fairness. See, e.g., U.S. Dept. of State, Country Reports on Human Rights Practices for 2003, Submitted to the House Committee on International Relations and the Senate Committee on Foreign Relations, 108th Cong., 2d Sess., 702—705, 1853, 2023 (Joint Comm. Print 2004) (describing failures of “due process” and citing examples in which “the testimony of one man equals that of two women”). And they would include a conviction for conduct that domestic law punishes far less severely. See, e.g., Singapore Vandalism Act, ch. 108, §§2, 3, III Statutes of Republic of Singapore p. 258 (imprisonment for up to three years for an act of vandalism). Thus, the key statutory phrase “convicted in any court of, a crime punishable by imprisonment for a term exceeding one year” somewhat less reliably identifies dangerous individuals for the purposes of U.S. law where foreign convictions, rather than domestic convictions, are at issue.

Thomas, Scalia, and Kennedy dissent, arguing that "any court" means any court, and foreign convictions can also include things such as murder, rape, and robbery.

I always thought that counting a foreign conviction as a felony might pose even bigger problems -- you might find that Resistance fighters against the Nazis were classified as prohibited persons, for example.