The idea of a gap between Gen1:1 and verse 2 is a comparatively recent understanding of Scripture but, until recently at least, was the default Christadelphian view. Thomas Chalmers, a Church of Scotland minister, (died in 1847) is credited as the man who popularised the idea of the gap, although the idea may have originated early in the 17th century. At the time geology was uncovering abundant evidence showing the planet was older than 6,000 years. This contradicted the traditional (and plain) reading of Genesis 1. In response, theologians and ministers proposed there was a time gap between v1 &2. The idea was that the heavens and earth were created long ages ago and anything older than 6,000 years was relating to previous life which was wiped out and everything remade as per the rest of Genesis 1. Consequently, it is reasonable to say the idea is not a traditional one, nor did it arise from the work of our community. The OEC position quite simply was informed by the science of the 1700 & the 1800s and is an attempt to match Scripture and science.
As previously discussed, our early pioneers, while rejecting evolution, engaged with and largely accepted the mainstream science of their day. As part of this, they accepted and promoted the idea of an old earth with a recent creation (OEC as it is commonly known). For example, in Elpis Israel, Bro Thomas wrote:
“Fragments, however, of the wreck of this pre-Adamic world have been brought to light by geological research, to the records of which we refer the reader, for a detailed account of its discoveries, with this remark, that its organic remains, coal fields, and strata, belong to the ages before the formation of man, rather than to the era of the creation, or the Noachic flood. This view of the matter will remove a host of difficulties, which have hitherto disturbed the harmony between the conclusions of geologists and the Mosaic account of the physical constitution of our globe”
Bro Thomas perceived “hints casually dropped in Scripture” (ibid) about previous beings on the earth – ie that the idea was certainly non-core (and part of his exposition included Jude 6 the angels in chains – which was, unfortunately, incorrect).
Bro Roberts similarly accepted the gap theory, writing:
“before the six days’ work began, he speaks of the earth as being “without form and void,” and “darkness on the face of the deep.”—(Gen. 1:2.) How long it had been in this state is not hinted; but the narrative leaves room for the measureless ages said to be required by geology.”
With the benefit of time, it is clear that the gap idea, or OEC, matches neither science (there is no break in the record of life) nor Scripture.
 Thomas, D. J. (1990). Elpis Israel: an exposition of the Kingdom of God (electronic ed., p. 11). Birmingham, UK: The Christadelphian.
 Roberts, R. The Christadelphian, Vol 10 page163 (1873)