Just to be more accurate, first of all, we must differentiate between 3 types of "gnosticism" that are very different
-early gnostics, that rose some time after the death of Jesus, they were some early christians and mostly jewish, they were considered heretics, they were a bit into esotericism, not necessarily mysticism (you could consider it mysticism in a very wide sense). Maybe these ones could be considered christian mystics. They were christians but considered heretic after the 2nd century. Later their doctrine was picked up by the Templars, but I think in a very convoluted way.
-Neo-gnosticism, there's some authors that consider themselves neo-gnostics or people who have re-started gnosticism based mainly in the old texts but also picking stuff from modern and comtemporary sources, also hinduist and oriental philosophy, some are into the occult somewhat. I would say it's not exactly mysticism but again, it depends on what you want to consider mysticism. They are not very close to christianism but much more eclectic. They do talk about sex-magick and things like that but for most authors is not a central concept/practice.
-pagan gnosticism, like theosophy or O.T.O. as considered Gnostic Catholic Church and they talk about gnosticism in a very specific way (19th century), very into the occult and very eclectic, closer to pagan (pre-christian and even pre-jewish) sources and oriental sources than to christianism, they talk about Christ but as an "Ascended master" so it's not like he is more important than Buda Gautama, for example. Sex-magick and other ceremonial magick it's very typical of them, specially the latter forms/group. They are not christians and you could consider them mystics but not christian mystics in any way.
Tradionally christian mysticism is linked to several figures: Origen, Pseudo-Dyonisus, Hildegard von Bingen, Meister Eckhart, Theresa of Ávila, Juan de Yepes...