GDP is a measure of total production in a period.
Q1's GDP cannot be cummulated in Q2's GDP calc.
you can think GDP is a country's income(salary, for example) rather than wealth.

GDP是流量概念，在一定程度上反映了当年新增多少财富。

流量（flow):一定时期内发生的变量

存量（stock):一定时点上存在的变量。

2楼的最后一句话切中要害