Operation Babel: New Tokyo Legacy

This is not cumulative, so you need to have 500,000 GP on you at once. If you change formation in CPA Headquarters, sometimes you see that you "lose" all of your GP, but your GP is actually put in storage. You can always store and retrieve GP in storage by going to Development Lab and then selecting "Growth Points Storage".

You don't even have to worry about this trophy. A post-game assignment "Embryo Core Accident" awards you with 1,000,000 GP upon completion (you might have to return to Headquarters to unlock).