To answer the OP's question, and this is straight from the website of Charles Bruni (the inventor of the Geo Method), no, it's not important if it's powdered or liquid Calgon. Charles Bruni states that he uses the powdered softener because it's cheaper by volume and weighs less by volume:
Liquid water softener, liquid laundry detergent, and liquid diswashing detergent all work too. The key points are water softener and detergent. I prefer powder because it's cheaper by volume and weighs less by volume. I have plenty of cheap water on hand to hydrate it and pitch it down the commode and drains.
He really explains the process in detail on his home page
There's also very good information on his blog that goes all the way back to 2005:
The Geo Method
Somebody made an entry in the blog about if it matters what kind (brand) of water softener is used and he claims that using a "non-precipitating" softener works the best with the Geo Method.
The brands not
to use: Arm and Hammer Washing Soda, Raindrops, Blue Dew, Borax, Climalene, Melo, White King Water Softener, and Borateem.
The brand you SHOULD
be using: Calgon
(a mixture of non-precipitating and precipitating chemicals), White Rain, Blue Raindrops, and Spring Rain
That information can be found toward the middle of the page of the link above titled The Geo Method