From the uncertainties in the voltage and current measurements, draw error bars on the data points in the graph.
On that graph draw a line which represents the 'best' fit to the data points.
Take that as your probable value for gradient and find the Boltzmann constant.
Then draw a second line which could reasonably pass through the data points and passes through the error bars.
Find the Boltzmann constant from this second line.
Take this as the upper or lower limit in the final result. (Depends where you drew the line)
Use this upper/lower limit to express the plus/minus in the final result.
Something along these lines. In the example here, an upper AND a lower limit was found.
This is also a possible way of doing this.
Depending on the level at which you are analysing errors, there are alternative methods, but this is pretty standard practice at A-Level