Well it depends on what you take to be "gnostic Christianity," because the current scholarly trend is to say that it never existed in the first place. Following much the same critique that scholars like R.I. More have applied to the cathari of the twelfth and thirteenth century, modern examinations of so-called "gnostic" groups have begun to recognize the tautology in which earlier scholarship was ensnared. That is, having been given the idea from "mainstream" primary sources that "gnostic" sects existed, earlier scholars went and found texts that matched (roughly) the beliefs described, declared them "gnostic", and then asserted them as proof for the existence of "gnosticism" as described in those primary sources! And so many scholars are now much more hesitant to reify categories and to postulate coherent and self-contained belief groups centered on particular texts.

If we use it as a more abstract term, we see many "gnostic" ideas - I would not use the word "theologies" as it implies a systematic structure - fade away while others are suppressed, depending on particular political circumstances and how opposed they are to the reigning orthodoxy or orthopraxy. But many of them are incorporated into the practices and texts of the mainstream Church - the Gospel of John and the Book of Revelation are both "gnostic" texts. Moreover, the interest in such "gnostic" ideas within the Church waxes and wanes over time; many aspects of later medieval monasticism might be termed "gnostic", for example.

If you want to read more, I'd recommend Karen King's What is Gnosticism?, which got this whole re-examination started.