Jin Yong's wuxia novels are so deeply embedded within the context of actual Chinese history that it's easy to identify the era in which each story unfolded. Anyone who is even reasonably familiar with Chinese history knows that DEMIGODS & SEMIDEVILS occurred during the Northern Sung Dynasty (A.D. 960-1127), LEGEND OF THE CONDOR HEROES and RETURN OF THE CONDOR HEROES took place during the Southern Sung Dynasty (1127-1279), HEAVEN SWORD & DRAGON SABRE during the Yuan Dynasty (1279-1368), STATE OF DIVINITY and CRIMSON SABRE during the Ming Dynasty (1368-1644), and DUKE OF MT. DEER, BOOK AND SWORD: GRATITUDE AND REVENGE, and FLYING FOX OF SNOWY MOUNTAIN during the Qing Dynasty (1644-1911).
With Gu Long, things are not as certain. Gu Long's novels never took place within a specific historical context; we only know that his stories occurred before modern times. TV and film adaptations of Gu Long's most popular novels depict the characters as definitely pre-Qing Dynasty (i.e. the characters are never shown in the characteristic pigtails and half-shaved heads of the Qing Dynasty era), but how far pre-Qing is uncertain. Such famous sects as Mo Dong Sect and Ngor Mei Sect are referenced in popular novels such as LUK SIU FUNG, CHOR LAU HEUNG, and the LITTLE LEE'S FLYING DAGGER series, and in Jin Yong's universe, that would peg the stories as Ming Dynasty (as there was no discussion of Mongol rule, and the emperor who appeared in LUK SIU FUNG was clearly a Han Chinese). Gu Long's Mo Dong and Ngor Mei, however, did not necessarily originate during the Yuan Dynasty as they did in Jin Yong's universe.
The most likely conclusion, however, is that all Gu Long stories took place during the Ming Dynasty.