Tillemont says either of Nyssa or Nazianzus.  In the ms. Coisl. I. it is preceded by lxxi., unquestionably addressed to Gregory of Nazianzus, and inscribed “to the same.”  In the Codex Harl. it is inscribed Γρηγορί& 251· ἑταίρῳ.  Garnier, however (Vit. S. Bas. xxxi. § iv.) allows that there are arguments in favor of Gregory of Nyssa.  Probably it is the elder Gregory who is addressed.  See Prolegomena.