Well, okay. This is going to send this thread straight to BTS, though. I can do it pretty easily through masturbation, but not so easily with sex. For beginners, you have to be really well hydrated. The easiest way to make this happen is to rub on the G-spot really firmly. Do it almost so firmly that it is somewhat uncomfortable, but don't come until you feel this pressure building up, similar to having to pee but not really. I always know when it is going to happen.
Another way to make it happen, is extremely intense clitoral vibration. Vibrators don't vibrate hard enough. Remember the post a few days back about sonic toothbrushes. Well, there was some double entendre there. That thing...well. Just use the back of it and do it pretty firmly right on your clitoris. More stimulation than you think you will even like at first, but that is sort of how "gushing" comes about.
Some girls are just more prone to do it than others, I think. I got one of my girlfriends to do the sonic thing, though, and she reported back that she gushed like crazy.
I have only been able to do it with one bf. I dated him for about 3 years, and he was probably 10" long. Just insane. Anyway, when I was on top I could rub a certain way that produced the best orgasms ever.
Try it. They feel better and harder than ho-hum orgasms.