I think it comes from the units in the definition of g:

(For example, if working on improving their tools currently offers the software developer the opportunity to improve their wealth creation skills at a rate of a 50% increase in their ability per year, their growth factor would be 1.5.) We'll call that g for growth.

At that point, you've accounted for current present value of your current skills, but now you want to add in the value of future growth. The '1' in '1.5' is what you have now, and is always what you have now by definition; the '.5' is additional growth. To remove what you already have, you do '-1'. Hence you multiply by 'g-1'