1. If the acronym you used, "IC" stands for "Internal Combustion" then there is no difference between an "IC engine" and a "Gas engine", they are both internal combustion engines.
These questions have a slight feel of home work to me. If this is home work the first answer is on me, you will have to do some reading on your own for the second answer.
Good Luck!
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There is no "unit of measurement of knocking" per se. But if you do some reading for your homework, you will find it has a lot to do with ignition timing among many other factors.
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