Someone who will send me Bitcoin insists I use a Gdax address from gdax.us, is there any reason for this I am not aware of ? like he wants to launder "dirty" bitcoins through Gdax using me or something ?
just like other posters say, it's definitely scam attempt using phishing/fake site
when someone demanding you to use certain site/wallet/exchange, you should be very cautious
receiving bitcoin only need a bitcoin address, requiring specific wallet is not normal
if you fall into his scheme, you will lose your bitcoin within minutes or days
he will steal it after he received whatever you're giving him in return of that bitcoin payment