Ghaz-E-Basar Meaning
This is what it is, to lower one gaze is a phrase to tell to adopt decency in your manner of looking at someone.
Because this is the way to show decency in manner of speaking. There is no rule that in a sentence both literary tools can be used at the same time. If I say my soon is the coolness of my eyes and I kiss him when I see him. The first one is metaphor and the second is literal.
In this very verse it is for decency in the manner of looking. To lower down one gaze includes the meaning of looking away and not to stare etc.