grammar of expressions

@AnyON has it right, the programming language in After Effects is a cut down version of Javascript.  Semicolons are used to end a statement in Javascript, as otherwise it reads as:

var TargetL = thisComp.layer("GradComp")[TargetL.width, TargetL.height]

Which AE would try to take as create a new variable, called TargetL and make it equal to a values in an array called thisComp.layer("GradComp") and those values are TargetL's width and height, which we've just made - in other words something not possible to process.