You are just unlucky. I am rank 150 and have gotten 15 free GP.

I'm rank 232. I have never received the free GP. I do have legion mates that have.
I have bought once $20 for 100GP I think to get rid of a bad planet mistake when I was below rank 50.
I still have 50GP left.
with your formulas I would have an extra 232GP or 58GP beyond what I have.
Why would I (or a player like myself) ever pay again?
I really wonder if it would increase the revenue to DAN.

What might work is an announcement that 4-10 times as many daily reward GP's will given out on X day which is typically low turn out. Like holiday's or weekends maybe when people are not in a routine. (or maybe those days have more active players).

Many games give 1 of whatever to players on leveling as a sampler, you can't do that much with it, but you can do something with it. It's so common in FB games I'm surprised it hasn't been used here. Although, it does mean if you play enough and save (unlikely for most ppl without foresight), you could have an exotic eventually. If you think also, "oh I am halfway to having that from free points, maybe I will buy the rest", that also makes a purchase more likely. Or, since they can get a couple unique things GP only, they might want some more. Thus, a sample is good. Or ... they might blow it on removing crappy planets, realizing an early game mistake, but it helps keep interest I think.