For the semantic development within Syriac from "to finish" > "to perfect", see S. Brock, "Fire from heaven: from Abel's sacrifice to the Eucharist: A theme in Syriac Christianity", Studia Patristica 25 (1993): 238 (reprinted in Fire from heaven (Ashgate Variorum, 2006).
Also, see note s.v. gmr #3.