there are some conditions to win credits (gold, as I understand it, is only for beginners, and no one needs it at all, galacredits you can spend in the store), they are not clearly stated anywhere, but based on my own observations, they are as follows:
[...]
sometimes challenges gives nothing at all, even if you have fulfilled these conditions.
sometimes challenges give more than one galacredit ($0.02 or $0.04), but the conditions are not very clear, apparently, time, points scored, and "work" done are taken into account.