When did "tithing" come to mean "giving 10% of your gross income to the Church"?

The Jewish Historian Alfred Edersheim seems to have done a fair bit of research on this subject. Basically, the subject is far more complex than a simple 10%, and would usually have amounted to more under Old Testament law. In fact, I am not sure I fully understand what Edersheim has explained—but that helps to convey just how complex it really was.

The basic idea of the tithe (i.e 10%) was bound up in the first fruit offerings of the land:

Two of these firstfruit offerings were public and national; viz. the first omer, on the second day of the Passover, and the wave-loaves at Pentecost. The other two kinds of ‘firstfruits’—or Reshith, ‘the first, the beginning’—were offered on the part of each family and of every individual who had possession in Israel, according to the Divine directions in Ex. 22:29; 23:19; 34:26; Numb. 15:20, 21; 18:12, 13; Deut. 18:4; and Deut. 26:2–11, where the ceremonial to be observed in the Sanctuary is also described. Authorities distinguish between the Biccurim (primitiva), or firstfruits offered in their natural state, and the Terumoth (primitae), brought not as raw products, but in a prepared state,—as flour, oil, wine, etc. The distinction is convenient, but not strictly correct, since the Terumoth also included vegetables and garden produce. (The Temple - Its Ministry and Services as they were at the time of Christ, by Alfred Edersheim (1825-1889), chap. 19, p. 379)

However, these first fruits and their offerings were complex in the traditional regulations and were largely based on being actually from the Holy Land:

They must be the produce of the Holy Land itself, in which, according to tradition, were included the ancient territories of Og and Sihon, as well as that part of Syria which David had subjugated. On the other hand, both the tithes and the Terumoth were also obligatory on Jews in Egypt, Babylon, Ammon, and Moab. The Biccurim were only presented in the Temple, and belonged to the priesthood there officiating at the time, while the Terumoth might be given to any priest in any part of the land. The Mishnah holds that, as according to Deut. 8:8 only the following seven were to be regarded as the produce of the Holy Land, from them alone Biccurim were due: viz. wheat, barley, grapes, figs, pomegranates, olives, and dates. If the distance of the offerer from Jerusalem was too great, the figs and grapes might be brought in a dried state. . . .

Of course, neither tithes, nor Biccurim, nor Terumoth, were to be given of what already belonged to the Lord, nor of what was not fairly the property of a person. Thus if only the trees, but not the land in which they grew, belonged to a man, he would not give firstfruits. (The Temple - Its Ministry and Services as they were at the time of Christ, by Alfred Edersheim (1825-1889), chap. 19, p. 379)

In the end, when all requirements were considered, the actual amount to be given was more than just the tithe, and would have been around 25% of the overall revenue of the community from the harvests that God caused to be enjoyed:

Thus the prescribed religious contributions of every Jewish layman at the time of the second Temple were as follows: Biccurim and Terumoth, say two per cent.; from the ‘first of the fleece,’ at least five shekels’ weight; from the ‘first of the dough,’ say four per cent.; ‘corners of the fields’ for the poor, say two per cent.; the first, or Levitical tithe, ten per cent.; the second, or festival tithe, to be used at the feasts in Jerusalem, and in the third and sixth years to be the ‘poor’s tithe,’ ten per cent.; the firstlings of all animals, either in kind or money-value; five shekels for every first-born son, provided he were the first child of his mother, and free of blemish; and the half-shekel of the Temple-tribute. Together, these amounted to certainly more than the fourth of the return which an agricultural population would have. And it is remarkable, that the Law seems to regard Israel as intended to be only an agricultural people—no contribution being provided for from trade or merchandise. Besides these prescribed, there were, of course, all manner of voluntary offerings, pious works, and, above all, the various sacrifices which each, according to his cirumstances or piety, would bring in the Temple at Jerusalem. (The Temple - Its Ministry and Services as they were at the time of Christ, by Alfred Edersheim (1825-1889), chap. 19, p. 379)

Edersheim makes it very clear that the Mosaic Laws did not apply to all income, and assumed an agricultural society, clearly dating some aspects of God's purposes for the Mosaic Law:

And it is remarkable, that the Law seems to regard Israel as intended to be only an agricultural people—no contribution being provided for from trade or merchandise. (The Temple - Its Ministry and Services as they were at the time of Christ, by Alfred Edersheim (1825-1889), chap. 19, p. 379)

When we look to the New Testament we can’t draw real parallels from the Old, for the intention was to support a government as well as the church. In a sense this means that under the Old Testament, once taxes and church offerings were paid, 75% of most peoples income was left. Of this, people could still make free will offerings.

As the original society under Moses in the desert would not have necessarily had any currency, most payments for other services were probably made with produce from the land: meats, etc. (although in some cases silver may have been in use). It is therefore somewhat natural that a tradition under the Old Testament of paying a generic 10% for the tithe on 'all that one earned' is understandable (Ref. Luke 18:12; Genesis 14:18-20), though not really arguable directly from scripture.

To answer the question more specifically by date, as it was first established within Christianity under Catholicism, I refer to a Catholic Encyclopedia that established it around 567 AD:

The payment of tithes was adopted from the Old Law, and early writers speak of it as a divine ordinance and an obligation of conscience. The earliest positive legislation on the subject seems to be contained in the letter of the bishops assembled at Tours in 567 and the canons of the Council of Maçon in 585. In course of time, we find the payment of tithes made obligatory by ecclesiastical enactments in all the countries of christendom.

By the "Old Law" the text means Mosaic Law. As far as the concept of gross versus net goes, this seems to be a more modern idea by hyper-fundamentalists, and I have not yet found its origin.