In the printer’s manuscript, scribe 2 misread the initial capital G of Gidgiddonah as an S, which would have looked somewhat like G in Oliver Cowdery’s hand (here Oliver is the presumed scribe in 𝓞). Later, in proofing 𝓟 against 𝓞, Oliver corrected scribe 2’s initial S to G. The 1830 edition has the G, so Gidgiddonah must be the reading of the original manuscript. In addition, there are ten occurrences of the related name Gidgiddoni in the early chapters of 3 Nephi.
Summary: Maintain in Mormon 6:13 the name Gidgiddonah (the 1830 reading as well as Oliver Cowdery’s corrected reading in 𝓟), not Sidgiddonah (scribe 2 of 𝓟’s misreading of the name).