Can a 20A GFCI be added to beginning of a 20A circuit that serves (8) 15A receptacles & light fixtures?
There's two problems I see here


There's probably no need for a 20A plug. Let's be honest, how often have you looked at your outlets and gone "Man, I really wish I had a 20A plug right here!" in your laundry room? Washers are 15A, gas dryers are 15A and electric dryers need a dedicated 30A two phase plug. What are you going to use that's 20A? Don't add 20A just for the sake of having 20A. For a shop or garage, maybe, but inside your home you'll rarely need anything more than 15A.
I would just GFCI the outlet you need, and 15A at that (remember, a 20A GFCI will be more expensive). Too many people think that they must use the LOAD side of their GFCI because it's serial wiring. Or we're wasting the plug somehow if it's only protecting that one outlet. NOT SO! Wire in series only those outlets that need a GFCI. If they don't need GFCI protection then just pigtail the connections and leave the GFCI out of the series.