Here in the original manuscript, Oliver Cowdery initially miswrote God as gd, which he immediately corrected by erasing the gd (and then partially crossing it out). But when Oliver came to copying from 𝓞 into 𝓟 (about six months later), he misread the erased gd as the indefinite article a. Thus 𝓟 and most of the earliest editions read “art thou convinced of the power of a God”. The 1840 edition emended the text here by removing the intrusive a, but very unlikely by reference to 𝓞; the reading with the extra a just seemed unacceptable. Similarly, the 1852 LDS edition removed the a from the LDS text, either independently or by reference to the 1840 edition. The current standard text correctly reads “art thou convinced of the power of God”, as will the critical text.
Summary: Maintain in Alma 30:51 the original reading, “the power of God”, rather than the incorrect “the power of a God” that was accidentally introduced into 𝓟 when Oliver Cowdery misread the erased gd in 𝓞 as a.