Hey stormline,
Ok, well how I learned to do it was, as you say, it just happened. Actually kind of a funny story. I spent four months of last year very ill with pneumonia, chest infections, bronchitis, etc etc. One night, whilst having sex with my husband, right at that pivotal moment, I coughed. And both of us looked at each other, and he said, what was that, and I said, I think I just gushed. So as we continued I just realised the juxtaposition of tension and relaxation required, and in the following couple of days I experimented with understanding the feeling and when precisely to let go, and now I have to actually work to hold back in certain circumstances.
As for "teaching", well, I must explain without breaking any confidences. The feeling right before you gush, bears some similarity to the feeling of needing to pee. I realised that for a long time I had been rushing off to the toilet immediately after sex, or sometimes actually stopping mid-way and going to the toilet, and when (what I thought was) peeing I would have these surprise orgasms. I don't think I was actually peeing, I think I was already gushing, but had been holding it back because I didn't want to pee on my partner.
When I realised that it's actually a uniquely different feeling of ecstatic pressure, and that the fluid that comes out is not pee (I know, I've tasted it), rather it is the sweetest without being sickly tasting liquid (not salty and thick like boy cum), it freed my inhibitions to experience orgasms completely differently to before.
And so whilst you can do the physical things with a girl that will get her to the point of gushing, if she is inhibited mentally in some areas, she may not be able to let go sufficiently to gush. That's the bit that really only a girl can help another girl with, because we understand the head space. That was the circumstance I enjoyed with a friend, was talking with her first and then creating a safe environment where she could relax enough to let it happen. I guess it's more explanation and then opportunity, rather than teaching.
Disclaimer: The aforementioned experiences have been mine alone and allow totally for other goddesses to have experienced it differently.
Love
swingingstories