hackers may choose our gmail account to hack our bitcoin account
They can only do that if you choose to use an online wallet service involving gmail as a means of logging in.
That's one of the major problems with an online wallet - they're reliant on the safety of the users' device
as well as requiring a significant amount of trust in a third party service (sometimes even allowing them access to private keys). An attacker with a keylogger could control your coins quite easily.
bitcoin is already very secure and I always use two factor authentication for logging into my account.
Those two things are entirely unrelated. You are relying on the security of a
third party service. It's not related to Bitcoin's security, only the way in which you decide to store your coins.
Regardless, two-factor authentication is vulnerable to a 5-dollar wrench attack. A thief could attack you and demand to know your code. Even though it's secure about online hackers, there are a variety of other possible methods for theft.