The Traveller setting for GURPS is in the year 1120 and there was no rebellion or virus. GURPS Space was needed for the game. The GURPS Traveller supplements that followed covered everything about the 3rd Imperium races, ships, and territories.
GURPS Space was most definitely NOT required. At all.
In fact, you could run a credible, fun, effective game using just the Traveller core book and GURPS 3e Lite. You didn't even have to buy the GURPS 3e Core book. GURPS 3e Lite was sufficient. (And far less fiddly, too.)
But, yes, GURPS Traveller followed an alternative timeline where Strephon was not assassinated. Instead, Dulinor's gig blew up on that fateful day, and never made it to the palace. Also, it started in 1120 or so, but by the end of the various supplements, I believe it made it to around 1128 or so.
Oh, forgot to add: At the end of the run, they published GURPS Traveller: Interstellar Wars. This was set in the years of the Interstellar Wars between the First Imperium and the nascent Terran Confederation. That was based on the newer GURPS 4e, and was a very nice presentation of Traveller and a new setting. It was very nice. Unfortunately, I guess it never sold well enough, as there were never any follow-on products for it. A shame, really, as it was a very nice setting.