I figure we have read many of the same texts. The "Gnostic" language is different from the opposition against it. What is the best way to represent the difference?

I will just refer back to those two links I provided the page before, about the definitions of 'noetic' and 'gnostic'.

As you already noted, Plotinus saw himself in opposition to the Gnostics, although in all fairness, from the perspective of today's world, the kinds of differences that they argued about are practically unintelligible in modern terms. (It's rather like many of the arcane arguments between Advaitins and Buddhists, who seem very similar to each other from a modern Western perspective, but who, in their own culture, saw themselves as worlds apart. There are many such arcane distinctions in these matters.)

In regards to the "constitution of the world", the argument Plotinus makes against the Gnostics can fairly be applied to Christianity, as it has come to be in its various expressions. Is this the best of all possible worlds or is it and us in in need of salvation?

Augustine may have appropriated the language of Plotinus to support a Monist view of nature but Plotinus would never have accepted the view that a City of God was struggling against the City of Man. In that respect, the term "Christian Platonist" is an oxymoron.

There was no "metaphysical world" at the time, that was an invention of Aristotle. For a man not 'for the people' he was remarkably loyal to them. Have you read Crito?