There's a good chance the exchange itself could have been compromised or another service you subscribe to. Usually it's pretty easy to tell if your gmail was compromised because of the extensive logging and security tools they have in place. Plus you had 2fa.

Seems really unlikely that a hacker would be able to specifically target your computer after knowing your involvement with BTC so we can rule out a targeted "blackbox" attack. We can also rule out session hijacking (stealing cookies) as he was able to get your password to other exchanges. You say you keep your system secure and updated so it's unlikely you were caught by a trojan or botnet.

You need to catalog ALL places where you have used that username/email and password combo and try to determine if one of those sites/services were compromised because that sounds like the most likely attack vector. Could be as simple as someone hacking a forum that you frequent, cracking your password, then trying it out to see what they can access.