Here's why:

* We haven't directly measured it: We can't send probes to exoplanets like Gliese 581c to measure its gravity directly.

* Estimating from mass and radius: Gravity depends on the mass and radius of a planet. We have estimates for Gliese 581c's mass and radius, but they are uncertain.

* Different models, different results: Different models used to calculate exoplanet properties lead to variations in estimated mass and radius, thus influencing the estimated gravity.

However, we can make some educated guesses:

* Gliese 581c is likely more massive than Earth: This suggests a higher gravity than Earth's.

* Estimates vary: Some studies suggest the gravity on Gliese 581c could be up to 1.6 times stronger than Earth's, while others suggest it might be closer to 2.2 times stronger.

In summary, while we can make educated guesses, we don't have a precise measurement of the gravity on Gliese 581c. More research and better observational data are needed for a definitive answer.