How to determine the distance of gaussian filter?

Hi everyone, I was confuse on how to determine the distance of gaussian filter. there are some article mentioned that the data filtered with ~20km gaussian filter. I wonder how they determine the distance of filter? Can anyone explain in more details.

This is just my example code and attach files.

clc; clear all; close all;

A = load('Example.txt');

x = A(:,2);
y = A(:,6);

Filty1 = imgaussian(y,3);
Filty2 = imgaussian(y,6);
plot(x,y,'.-b'); hold on;
plot(x,Filty1,'.-r'); hold on;
plot(x,Filty2,'.-g'); hold on;