i have received iphone 4s as a gift and want to know if its a genuine piece serial number of my phone is C39GTBXSDT9Y

punch in your serial number here: https://selfsolve.apple.com/agreementWarrantyDynamic.do

Based on the result I got, it was either replaced and then re-sold illegally or flagged as stolen / lost.

hello, what would happen if it was being replaced? can you still use the phone?

Sure. Just like you can still drive a stolen car.

just like how the real owner can? I mean like i can still use the apple store, run some updates, download some stuffs and the likes?