The gastrophetes is a well documented piece of equipment, and I believe it would have been just as powerful as a contemporary Chinese weapon.  The Chinese used self prods as well as laminate and composite ones, and the only differences between gastrophetes and Chinese crossbows are in the mechanisms, not in the strength of the bow, it's efficiency, etc.  I expect a gastrophetes would be a quicker weapon to use too, because of the manner in which it was spanned, as opposed to the Chinese method of getting down on the floor and pulling with all your might, or using a long, thick rope stirrup.