Quick answer
The measurable difference
Giganotosaurus leads this display by about half a metre in length. The very large T. rex is roughly 1.2 tonnes heavier and more robust.
Why this matchup matters
Different bodies, not just different numbers
Their total lengths are close enough that skull shape, torso depth and mass matter more than a simple nose-to-tail ranking.
Human scale
How the gap feels
Half a metre is less than one third of the reference person's height. The more visible difference is the volume concentrated through the T. rex chest, hips and skull.
Evidence boundary
What the comparison cannot prove
Giganotosaurus is incomplete and T. rex varies by specimen. They also lived on different continents and were separated in time, so no direct encounter occurred.