Nah, it started in December 2003. It was halted to an extent in November 2004 for 3 or 4 months, but was then started in earnest once more in July 2005, and only briefly abated in The Ashes 2006\07.
There had been no period, 1999\2000-2003, where Gilchrist had such a poor run of scores as he did December03-November-04. Gilchrist was no longer the (Test) player he had been after the Caribbean tour of 2003 was over.
Ha, you are going to find trouble for these selective stats dawg. Now listening to this i am convinced you didn't see Gilly bat during this period & you probably just resarched his career & came up with this.
As a person who has seen all of Gilchrist's 96 test live except for his two series in NZ in 99/00 & 2004/05 i'm a tell ya watch really poppin from what i saw not to selective stats..
Firstly just because a player starts having a few poor scores means he is going into decline which is where you are wrong about Gilchrist's decline. A player could be said to be in decline when he looks totally the shadow of the player he was at his peak & can no longer due to too much cricket (which caused a temporary loss of form to Tendulkar which lead to speculation of decline between WI 2002 to SA 2007) or players like Steve Waugh or Richards who declined due to age.
With Gilly now his decline started in England since that was the series where his weakness was found out i.e bowling around the wicket to him other bowlers around the world for the last 2 1/2 years tried, throw in the age factor & he wasn't his consistent best but still produced moments of his brilliance i.e 91 vs World XI, 86 vs SA SCG, 144 vs BAN, 100 vs ENG perth.
Your idea that it began way back vs IND in 2003/04 is foolish because if you watched that series yea he didn't get big scores but that was due more to him getting himself you by playing too aggressively & a him getting a few good fantastic deliveries i.e Pathan @ Sydney rather than him going into decline. I would consider his series vs IND in 2003/04 a bit unlucky.
As 2004 began he had two tours to SRI & IND countries that would expose his weakness againts spin that was always prevalent even during the 1999-2003 period that you mentioned it just wasn't tested then & given his weakness his batting in those two series especially in IND where he was skipper was as good as it could have possibly done.
In between that their was also the home series in AUS in darwin & cairns where in the first test he scored brilliant 90 odd on a very difficult Darwin wicket, then post IND he had field days vs NZ home & away & PAK before he went to ENG for the workout.
So again Gilly declined as a batsman began in the Ashes & not a series sooner.
This should cover you for now at least..