Hey raff5184,
from my understanding the value 27k is basically put as an example of calculating the rest of resistances and the whole process of adjusting values to those accessible on market.
If you compare it to E24 Series, resistor values like 21k, 51k, 270R and so on are widely available from any online shop or Ebay.
So simply speaking: it was used as an example for calculations. You could put any value for Rg and calculate the rest as shown.
Regards