You switched the operands in your assign statement.
Switch this
Math.abs(a[i]-a[i-1]) = biggestGap;
to this
biggestGap = Math.abs(a[i]-a[i-1]);
Math.abs(a[i]-a[i-1])
returns just an int value (no variable reference or similar). So your trying to assign a new value to a value. Which is not possible. You can just assign a new value to a variable.