You have to pay 25% tax to Canada if you get your CPP/Pension and go back to China.
CPP is mandatory in most of case.
I mean CPP contributions. As long as you get paycheque from your employer and your age is between 18 and 70, you have to contribute to CPP. Only self-employees have choice not to contribute.
CPP payment calculation is based on your annualized earning and years of contributions. That's not adjusted by other retirement income.
But GIS is based on how much the retirement income is. You will not be eligible to get GIS as long as you have CPP in most of case.
But you have to take consider of family income. GIS is not easy to get.