Supposing that you joined WGT on Jan. 1st, 2010 and collected 150 credits per day from day 1, you could've accumulated 572,100 credits, largely overpassing the amount you need to purchase the GP. Too bad...
But don't you worry. The beginning is half of the whole so let's establish a long-term plan to make your dream come true :
400,000 / 150 = 2,666.666666... Okay, 2,667 days.
If you start collecting 150 free credits immediately from today, you'll be able to buy yourself a GP on October 1st, 2027. As you will certainly get better and better at this, you might win some prize money here and there so your journey might be much shorter than expected.
At the end of the day, a man must carve out his own path. Good luck to you.