When you say "details" I don't know if that includes your username/password to online banking. If it does, first step is change that password, but you should do that while you're on the phone. Changing your password is not enough, you must also do the next step.
Tell your bank. Right now.
Call them on their customer support line. Don't wait for an opportunity to visit a branch.
Because they can provide practical defenses about what is about to happen. Or, if it's already happened, time is of the essence in reversing it.
The more proactive you are, the more likely you'll get your money back.
Then, do mop-up activities
Notify Google that the mail is a scam, Gmail provides a way to report that.
File a police report. They may not want to take your report unless the bad guy actually tried a transfer.
I am breaking these out into "mop-up" to emphasize that these are not as urgent, and don't do them first.