For the line vs the overall Traveller canon? Because the license was stated to be non-authoritative. By both Marc and Steve Jackson. That said, Marc prefers that it not be contradicted.
For the GURPS Traveller line internally, Almost all of it is canon, but a few items by one author shouldn't be; he published them first for Spacemaster, and thus shouldn't be considered, because they cannot be used elsewhere in Traveller, and are poor fits even for GT. GURPS Vehicles is also canonical for the GURPS Traveller line; GV is the underpinning of the GT ship building system.