The chia gel is actually just chia seeds that have been soaked in a liquid. You will get the same reaction if you put chia seeds in a NutriBlast and allow it to sit for a little while. This is why the NutriBlast gets thicker when you use chia seeds.
That being said. You can add the gel in either before or after - the same benefits will be achieved either way! If using plain chia seeds, add those before Blasting so it gives them time to gelatinize.