How do you decide what Gnosis to give the party? I've experimented with varying numbers and found it to change the dynamics drastically, despite the players not knowing what number I've assigned them.

So, is there a rule of thumb that other GMs have used, or is it just up to each individual GM to figure out?

I always give a base level of 5 or 10, as recommended in the base book. Anything more, and they gain access to creature powers, as well as open rolls and Destiny Points from Natura, as described in Gaia.

Thanks, but doesn't that put the cap on potential pretty low as well?

Not at all. I am not totally sure I can explain it well without rereading, but I suggest that you re-look at the Gnosis and Natura sections. They shed a lot of light on the subject.