I don't see much about T-Rex being heavier. The sources I see generally put T-Rex around 5-7 tons and Giganotosaurus around 6-8 tons, though one source, I think, put it around 5 tons, but that's it. Another source or two say T-Rex was heavier, but don't give a specified weight or anything. Another source says the results likely varied depending on individuals, meaning some T-Rexes could have been heavier than Giganotosauruses, and some Giganotosauruses could have been heavier than T-Rexes