The ResultIndex function of a GradientColoring formula returns a value for #index, an index into the gradient. The #index has useful values from 0 to 1. If #index is outside [0,1], 0 is used.
If Repeat Gradient is checked, positive values greater than one are reduced to [0,1]. For example, 3.7 becomes 0.7, which is in range. However, negative values of #index are still replaced by 0.
It would be reasonable for negative values also to be reduced to the [0,1] range if Repeat Gradient is checked. Then -3.7 would become 0.3, in range.