Different weapons, though, I'm afraid. My understanding is that the Han crossbow was a hand-held, more "modern" crossbow, such as the type we generally think of existing in the European Middle Ages. The ancient Greek version was a larger, more siege-oriented one that would be quite bulky and unwieldy in a pitched battle. Consequently, I would assume that the gastrophete probably packed a much bigger punch, but also caused the user to be very vulnerable, which is why they were more likely used in sieges.
This is just a general sense of it, so if someone knows otherwise and/or has more specificity, please say so; mine is a very minimal knowledge, so a more studied "expert" would be most welcome.
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