Does the yoyo have to be spinning at a certain speed to do gyroscopic flop?
No, but the faster the better 
Yup ^^^
What they said. If the yoyo isn’t at a minimum speed, it won’t flop, it’ll just stall out.
Two things to know about Gyro flop:
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No two yoyos flop the same, although once you know the trick you should be able to hit it on pretty much any yoyo
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The strings must not come absolutely straight out from the axle. An angle is required. So if your yoyo is just spinning there and not flopping, try moving your NTH back and your TH forward just a smidge.
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