OK -- I design stuff like this but I don't work for Castle. So I've seen just about everything that can possibly go wrong and I'm deeply suspicious of any pie that I don't have a finger in, too boot.
And I don't even diddle with electric control line planes, so I have no direct experience. But if Castle isn't completely mistaken or lying about what those names mean then there's only so much that the behavior can deviate from normal industrial motor drive practice.
Now that you have my disclaimer, here's what I know:
Timing and PWM frequency should mostly have an effect on the overall system efficiency and shouldn't make a bit of difference to surging. However, if they're way wrong then they could have an effect. Normally I would expect that setting the timing or the PWM frequency wrong would make the motor run hot, and that heat would just be Joules that went to heating up the air instead of turning a prop. But I wouldn't expect that it'd significantly change the regulation of the speed loop, which is what the gain setting should do. If the timing was so bad that you were actually making the motor wimpy then it could affect surging -- but it's unlikely. Ditto PWM frequency.
Gain should have the most impact on the surging, if the gain setting and the surging are both what I think they are. In an ideal world (i.e., one in which timing and PWM frequency never made weird things happen) then gain would be the only thing you'd have to worry about.
Stick a prop on the thing. Increase the gain until you notice definite signs of surging, then back the gain off by a factor of two. If their software is written the way I think it is, you should be fine.
I would look for signs of surging by grabbing the spinner of a running motor briefly with my gloved hand to slow the motor down, then letting it go as crisply as I could while listening to the sound it made as it recovered speed. Exactly one "waa" out of it as it speeds up, or as it overspeeds and recovers, is what I would expect. If it goes "waa waa waa", repeatedly speeding up and slowing down, then I would reduce the gain.
Needless to say, in order for that test to work you need to have the motor firmly mounted to a plane or a test stand, you need a prop and spinner on it, and if you're picky you need to use a spinner you don't care much about (if you're a lawyer you can put polishing compound on your glove, and say you meant to do that).