If you receive an income, someone had to spend money (in this case your employer). So it does in fact contribute to GDP, as your employer spent money buying your service/finished work.
So income you received already contributed to GDP, even if it just sits in your bank account. Now if you go onto spend it, it will further increase GDP, as you spending it will yield an income for someone else. If you leave it in the bank account, it will not further increase GDP, as it won't yield anyone an income.
Additionally, when a US citizen buys a Chinese good, it does in fact contribute to GDP - namely China's GDP.
If it's not clear to you by now, GDP is almost the same exact thing as money velocity, just divide it by the money supply and you have the velocity of money.