The main difference between the two is the laser focus on the pre- and postnatal period versus broader information covering a woman's lifespan.
CPPC covers everything a health and fitness professional needs to know from a coaching, psychology, nutrition, exercise, and rest and recovery perspective for prenatal and postnatal women.
GGS-1 covers coaching, psychology, nutrition, exercise, and rest and recovery in adult women across their lifespan, including sex differences and how they do (or don't) affect coaching and programming.
I'm including a visual that illustrates the difference between the Anatomy & Physiology sections of the two certification as I think it's a great representation of how the information differs.
There's less than a 10% overlap in the content between the two certifications. The only content that overlaps is content that MUST overlap, like the muscles of the pelvic floor, or lists of food that are high in protein, etc.
The rest of the content is incredibly pre- and postnatal specific. And since 85% of women in the U.S. will have a baby at some point (that number is even higher in other countries) -- if you train women, this content is incredibly important.
Once a woman has a baby, she's postnatal forever. Not in the way that she's considered "delicate" or should be treated with kid gloves. But rather, her body has been through events that must be considered when training her. Plenty of women return to very intense exercise after they have children, but knowing how to help them get there safely (if that's a goal of theirs) requires specialized knowledge.
A few other important things to consider