How can the noble verse "Say, 'If there were in the earth angels walking about in peace, We would have sent down to them from the heaven an angel as a messenger'" be reconciled with Gabriel's appearing in human form?

Ibn Ashour stated that the verse means Allah sends a messenger from the same kind of people to enable interaction. If the inhabitants of the earth were angels, Allah would have sent down an angel as a messenger to them. But since they are humans, He sent a human like them to them.

As for Al-Qurtubi, he explains that humans cannot bear to see an angel in its true form, and that if Allah had sent an angel, it would have taken the form of a man so that people could feel comfortable with him, because every species feels comfortable with its own kind and recoils from other species. This explains Gabriel's descent in the form of a human.