An adjective appearing as G. {gaib >>} gaiw “pregnant” in the Gnomish Lexicon of the 1910s (GL/37), derived from primitive ᴱ✶gīwă with the Gnomish sound change of ī > ai (GG/14).
Neo-Sindarin: I would adapt this word as ᴺS. gaew “pregnant” representing the Sindarin sound change of ai > ae, derived from the Neo-Root ᴺ√GIW having to do with pregnancy. In Tolkien’s later conception of the languages, its primitive form would need to be ✱gaiwa, the result of a-fortification of the root: compare [N.] maew “gull” derived from the root ᴹ√MIW (Ety/MIW).
A verb appearing as G. gaibra- “to impregnate” in the Gnomish Lexicon of the 1910s, a verb form of G. gaib “pregnant” (GL/37), likely related to primitive ᴱ√giu̯i (GL/39). Elsewhere in this document there was another verb G. gwectha- based on G. gweg “man”, which meant “impregnate” when used with the allative and “generate” when used with the accusative.
Neo-Sindarin: I posit the existence of Neo-Root ᴺ√GIW to serve as the basis for pregnancy words, so I would adapt the above verbs as causative ᴺS. gýda- “to impregnate, make pregnant” < ✱giu̯-tā-, with iu > ȳ as usual in Sindarin.