This is not required by Gamemaker, and will not produce any errors if you omit a semi colon, but it's good coding practise as other engines require it.The other thing... make sure you put the semi-colons after writing your lines, that could cause issues with the system reading the code seeing as how semi-colons tell the system that the line has stopped.
Not quite true... It is required for some functions and expressions to be parsed correctly, and the YYC can be very finicky if they aren't included. In general I would 100% recommend always using the semi-colon, even if you can "get away" without using it. It also future-proofs the code, as GML is constantly evolving and I can see it being made mandatory in the future to bring GML more in line with other popular languages.This is not required by Gamemaker, and will not produce any errors if you omit a semi colon, but it's good coding practise as other engines require it.