Hello Marcelo11756,
Here's my answer. Unfortunately, you can not calculate Average in the grid and show at the bottom of the column on that grid. The only option to calculate on columns is Subtotal. However, you can calculate the average of the column on the grid and dispaly it on the screen(not in the grid).
In order to achieve that, you have to assign actions "On Screen Load". You can start loop through the grid and get the sum of the column and record counts and assign them to action result, then use Basic Math calculation action to calculate the average and then enter the value on the screen.
Hope this helps!
-Inder