First of all, the detector tube contains a detecting regent (as already explained in the answer to the previous question: "Why does the gas detector tube change colour?").
Now, let us use the oxygen detector tube as an example. It contains tightly packed grains of the reagent powder as shown in the drawing below.

During measuring, air is drawn (sucked) into the detector tube using a sampling pump. The oxygen molecules in the air adhere to the grains of the reagent causing them to change from black to white.

One row at a time, the oxygen molecules advance through the reagent grains adhering to them as it penetrates deeper and deeper into the tube. This process continues, layer for layer, until there is no more oxygen and the colour change stops.

Essentially, differing gas concentrations mean different amounts of oxygen in the air that is drawn (sucked) through the detector tube with the sampling pump. When there is lots of oxygen (i.e. a high concentration), the oxygen molecules advance very far through the reagent (leaving a long strip of white), whereas when there is little (i.e. low concentration) it is quickly used up as it passes through and reacts with the reagent; so, the strip of changed colour (white) tends to be shorter.

Hence, the length of the discoloured strip in the detector tube, depends on the concentration of target gas in the air to be tested/measured.