Here we have another typo in the 1841 British edition that shows considerable neglect or sleepiness on the part of the typesetter. He apparently started to set gathered, but when he got to setting the final d, he set the ending -ing instead, thus producing the bizarre gathereing, which was never corrected in proofing (if it was even proofed). The typesetter’s error was probably caused by the occurrence of the present-participial form gathering in the previous verse: “wherefore they were for the space of four years gathering together the people” (Ether 15:14). Not surprisingly, the 1849 LDS edition corrected this typo. Here in verse 15, the critical text will, of course, maintain the correct gathered.
Summary: Maintain the past-tense gathered in Ether 15:15, the reading of the earliest textual sources.