The only patristic commentary I could find on Gal 3:1b comes from St. John Chrysostom:
Yet was He not crucified in Galatia, but at Jerusalem. His reason for saying, “among you,” is to declare the power of faith to see events which are at a distance. He says not, “crucified,” but, “openly set forth crucified,” signifying that by the eye of faith they saw more distinctly than some who were present as spectators. For many of the latter received no benefit, but the former, who were not eye-witnesses, yet saw it by faith more clearly. These words convey both praise and blame; praise, for their implicit acceptance of the truth; blame, because Him whom they had seen, for their sakes, stripped naked, transfixed, nailed to the cross, spit upon, mocked, fed with vinegar, upbraided by thieves, pierced with a spear; (for all this is implied in the words, “openly set forth, crucified,”) Him had they left, and betaken themselves to the Law, unshamed by any of those sufferings. Here observe how Paul, leaving all mention of heaven, earth, and sea, every where preaches the power of Christ, bearing about as he did, and holding up His cross: for this is the sum of the Divine love toward us.