The 1879 LDS edition placed the infinitive phrase “to get gain” (originally in the archaic or colloquial form “for to get gain”) closer to the verb say. With the original placement, there is the possibility of misinterpreting this infinitive phrase as modifying the immediately preceding that-clause (that is, “there can be no miracle wrought by Jesus Christ (for) to get gain”), which is clearly not the meaning of the text. In the early editions, a comma was used to separate off “(for) to get gain”, which helps the reader make the correct interpretation:
It is worth noting that in the RLDS text this emendation in word order has not been made. The 1830 punctuation, however, has the unusual use of a comma right before the that-clause (“say at that day, that … ”). One possibility is to remove that comma but leave the one at the end of the that-clause (or to replace it with a dash):
There are quite a few instances in the original text where phrases, usually prepositional, are displaced from where they are normally expected in English. For some discussion, see under Mosiah 26:23. For a more extensive list, see under displaced prepositional phrases in volume 3. Here in 3 Nephi 29:7, the critical text will restore the infinitive phrase to its original position, after the that-clause, along with its original for. For discussion of the infinitival form “for to do something”, see under Mosiah 20:1 or, more generally, under for to in volume 3.
Summary: Restore in 3 Nephi 29:7 the original infinitive phrase “for to get gain”—and in its original, displaced position after the that-clause; punctuation can be used to help the reader parse this difficult sentence; other instances of displaced phrases, usually prepositional, are fairly common elsewhere in the text.