When it comes down to a canon debate in terms of 'movies-only' criteria, GL's own Jedi/Sith backstory (expressed in the TPM novelisation and interviews around the same time) only extends a thousand years prior to the PT - Darth Bane & the Rule of Two etc. Beyond that, you have to go into very vague draft concepts that he came up with and handed over to EU authors, which were changed dramatically. GL originally envisioned 'the Sith' as a band of mercenaries taken under the wing of an exiled Dark Jedi by the name of Darklighter, who were then taught the ways of the Dark Side of the Force (not unlike the Mandalorians in the original Tales of the Jedi DH comics). In the EU, however, this morphed into the Sith being an actual race. According to Kevin J Anderson, who was involved in TOTJ and the Dark Empire series, he was provided with background on the Sith by GL, but I suspect it was just old notes & drafts for SW/ANH. IMHO, I don't believe GL had any concrete ideas about galactic history any earlier than about a thousand years prior to TPM, although there's a few suggestions here and there that he once may intended to have addressed such matters at a later date - back in 1978, when he was planning to churn out SW movies every year or two. What was established somewhat definitively by Obi-Wan's line in SW/ANH is that the Republic and the Jedi had been around for approximately 20-25,000 years. As far as what happened during that time, apart from a massive conflict about 1000 years before the Battle of Naboo, is anyone's guess. It's in the hands of the EU.