Well, I know I can cum from a lot of different stimuli. My husband can play with my clit, or chew and suck on my labia, sometimes during vaginal he'll miss my spot but I'll ejaculate anyway. Anal too if we're really turned on. My first orgasm with him was actually achieved through breast play, he kept massaging my breasts and tweaking my nipples and I ended up cumming right there. I know it was cum, because I gush whenever I climax, sooo..
Anyway yeah, g-spot stimulation is only one of many possibilities, though it really depends on the girl. I seem to be very sensitive all over so no matter what my husband does, I love it and it turns me on, but other girls might not have such high sensitivities to certain areas. The clitoris and g-spot are the easiest erogenous zones because they're really clusters of nerve endings.