The most recent official Smogon Tour round to feature GSC (and which explicitly published rules banning HP Legends) seems to have been this from 2007:
https://www.smogon.com/forums/threads/season-5-week-9-gsc-2-won-by-fear.19023/ Funnily enough,
Fear won it :) Sometimes I forget he was at this long before I got started, because he went on to kick ass long after I retired.
For some reason my brain wanted to put the lifting of the ban at 2009. This thread seems to indicate it was still around at the beginning of that year:
https://www.smogon.com/forums/threads/why-is-hp-on-legendaries-banned-in-gsc.50695/
However, that is probably not accurate. I think the decision might've actually been made in a thread or discussion somewhere about whether it would be allowed for the first SPL (which had signups in April 2010, began play in May 2010). They definitely were allowed for that first SPL, by the way, as this playlist of matches can attest:
I'm not sure where to find that discussion, unfortunately. The best historical throughline I can find for when the ruleset had "officially" changed, which lines up with the "changed for SPL" theory, is this:
gorgie posts an interest probe thread for a tournament featuring GSC in March 2010. In it, HP Legends are explicitly banned:
https://www.smogon.com/forums/threads/rby-gsc-adv-ou-mashup-tournament-interested.68159/
Then, a few months later, the actual signups for that tournament posted in June 2010 say they're allowed now due to recent changes to official tournament rules!
https://www.smogon.com/forums/threads/rby-gsc-adv-ou-mashup-sign-ups.72663/
This would be honored in the first round of the tournament posted days later:
https://www.smogon.com/forums/threads/rby-gsc-adv-ou-mashup-round-1.72807/
I want to say Pokemon Online allowing it may have contributed, too, but that might be bad chronology because I think GSC support on PO only really took off a year later. Alas, PO only has a couple patchy snapshots on the Wayback Machine remaining, so those records are pretty much lost.
Anyway, yeah, enough rambling. All signs point to them being officially unbanned by Smogon for the start of SPL 1 in April/May of 2010. It'd be cool if somebody smarter could post a more direct source for that than my guesswork though.