What was the socioeconomic makeup of early Christian Gnostic sects?

Most of what we know about the socio-economic makeup of early Gnostic groups is speculative. Their literature was often mystical and contemplative, affirming an other-worldly mythology in which the soul is trapped in the body, the physical universe is either an illusion or evil, and salvation is a matter of enlightenment that has nothing to do with physical resurrection. "You shall know the truth and the truth shall make you free," but not "Jesus died for our sins so that we may attain eternal life through faith in his atoning death and resurrection."

Gnostic literature was often mystical and filled intellectual-spiritual jargon having to do with spiritual entities in the Pleroma, their generation and relationship to the physical world. 1 Timothy apparently refers to this when speaks of advising its recipient to:

remain on at Ephesus so that you would instruct certain people not to teach strange doctrines, nor to pay attention to myths and endless genealogies, which give rise to useless speculation. (1 Tim 1:3-5)

We can infer from this and the character of gnostic literature that it appealed more to the educated classes, who had more leisure to entertain speculative philosophy than the masses, who could more easily relate to simple kerygma of the proto-orthodox church.

Irenaeus' praise of 'barbarian' Christians

Along the same lines, Irenaeus of Lyons spoke highly of illiterate Christians whose simple faith pleased God because they were not tempted by "portentous language" of heretics.

Many nations of those barbarians who believe in Christ do assent, having salvation written in their hearts by the Spirit, without paper or ink... [They] are barbarians so far as regards our language; but as regards doctrine, manner, and tenor of life, they are, because of faith, very wise indeed... If anyone were to preach to these men the inventions of the heretics, speaking to them in their own language, they would at once stop their ears, and flee as far off as possible [rather than] suffer their mind to conceive anything of the portentous language of these teachers. (Against Heresies: Book III, Chapter 4)

The OP asks: "Was their membership also very inclusive, or more limited to certain areas, such as cities rather than rural areas, or primarily made up of a certain class, gender, educational background."

Cities vs. rural areas

See above regarding educational background and class. We do know that Gnosticism was popular in cities. The proto-gnostic teacher Marcion was a wealthy leader in the Roman church, where he attracted a significant following. The eminent gnostic teacher Valentinus had been a candidate for bishop in that city, where he later maintained a school to disseminate his doctrine. His movement spread to Northwest Africa, Egypt, Asia Minor and Syria. Cities were probably more fertile ground for Gnosticism than rural areas, which had few centers of learning where people were likely to be attracted to new ideas and philosophical speculation.

Femininity in theology and practice

About gender, the gnostic groups seem to have had various attitudes. Several gnostic texts portray women as teachers, especially Mary Magdalene, who sometimes is the one disciple who truly understands Jesus' secret teachings. Several gnostic works speak of the Holy Spirit as feminine. According to Elaine Pagels in her ground-breaking book The Gnostic Gospels, women could achieve leadership roles in some gnostic churches, though not others.

To summarize: very little of a direct nature is known about the demographics of early Gnosticism. However, we can glean some speculative conclusions from their writings and what is said about them by their critics. They were likely to be literate and thus were mainly drawn from the upper classes. They were known to have made inroads in cities, probably more so than in rural areas. They were more likely than orthodox churches to include women in leadership.