Gold Kryptonite's first appearance, in an imaginary story, was in Adventure Comics #299.
This story establishes that 18 Kryptonian sun-cycles is about 25 Earth years.
Character(s):
Legion of Super-Heroes [Lightning Lad; Cosmic Boy]; Supergirl; Superman; Lana Lang; Bizarro #1; Lori Lemaris; Stephen Snapinn (intro); Comet the Super-Horse (cameo); Krypto (cameo); Lois Lane; Lana Lang; Jimmy Olsen; Perry White; Luma Lynai (flashback); Lex Luthor (flashback)