The original manuscript is sufficiently extant here that we can determine there was no of between the gerund saving and the direct object his people. The definite article the that precedes saving is not extant in 𝓞, but spacing between surviving fragments supports its occurrence in 𝓞. When the text was copied from 𝓞 into 𝓟, Oliver Cowdery inserted the preposition of: “he delighted in the saving of his people”. Alternatively, Oliver could have deleted the the: “he delighted in saving his people” (which is what we expect in modern English). Nonetheless, the original text had a number of mixed gerundives like the original one here in Alma 55:19, with the preceding the gerund but without any following of. For several examples that continue in the text, see under 1 Nephi 17:32 and Alma 40:15. Also see the general discussion under gerundives in volume 3; there I provide several examples of this usage from various sources. Here is one example from Benjamin Franklin dating from 1782: “He said there was no Want of Money in the Nation; that the chief Difficulty lay in the finding out new Taxes to raise it” (original accidentals retained).
Summary: Accept in Alma 55:19 the difficult reading in 𝓞 with the the before saving but no of between saving and his people; this kind of gerundive construction occurs a number of times in the original text of the Book of Mormon and will be maintained or restored wherever the earliest textual sources support it.