Greatest gift? I think it unlikely. In the first place, I don’t agree to a presumption that there is a giver of gifts on the scale of all humanity. As a line of poetry it has its points and might be very effective in a given context. I don’t recognize the aphorism (other than to see via Google that it is recycled from wis.dm). Is it a quote?
But if the actual question here is about recognizing the self within the other, and specifically about recognizing self within an antagonistic other, yes, I think there is great value in that. I think it is one of the most powerful kinds of teaching there are, provided that one is open to the lessons.
The reason I say this is that there have been occasions throughout my own life in which I have felt a strong aversion to someone, much stronger than dislike—a visceral reaction that bypassed all rational faculties—in situations where I simply could not avoid them, such as the workplace. I came to realize, usually not all at once but over time, that to me those people seemed to be gross magnifications of my own worst traits, the things I liked least in myself and wished I could banish. Dealing with them was exactly like confronting the darkest and most unpleasant corners of my own personal sewer. Their proximity made me feel exposed, as though anyone could see the invisible bond of likeness between us and their sheer presence in my vicinity would cause others to discover in me things I would prefer to conceal.
The people who talk about “awareness” with true understanding are not enamored of its unmitigated wonderfulness. They know it can be sharp, painful, even paralyzing before it is liberating.
Seeing these grotesque exaggerations of my faults in a disfiguring mirror was a more powerful shaper of behavior than any amount of therapy could have been. I never spoke of them to anyone, but I hold those teaching images in my mind almost like gargoyles meant to ward off evil spirits.
I wonder if I will ever be brave enough one day to ask myself to whom I might be performing the same admonitory service.
Gatekeeper? evolutionary step? I don’t know. I tend to think those things are accomplished one person at a time and not in any wholesale shepherding of minds into a new becoming. I won’t live long enough to complete the task of improving myself, but I do hope I make good use of the lessons I find in my path.