Your bitcoin wallet used as of now has been compromised, if you can get back your Gmail that has been hacked, just do it. But first, transfer your bitcoin into another wallet if you can because your fund now is not safe at all. The hacker will I guess contact the support team of the wallet and ask permission to withdraw your fund using other methods that allow users to withdraw the fund.
This is the problem if you will use a custodial wallet, the best thing is a fully open-source wallet that surely you are the only one who holds the private key.
Always remember what great holders says, if you do not own your private key, it means that is not your coin.
He probably has the access to his email account but wondering if he can bypass the 2FA authentication (I hope he enables it). This gonna be the last hope for the OP to get his crypto on a certain exchange otherwise, it all transfer to the other hands. But ain't that sure if the hackers got full control over his phone, access the 2FA after realizing that he got hacked. I'm afraid that non left for him.
This is the hardest part in crypto where to fully keep safe our funds and all our credentials. Anyone can be a victim and maybe we are lucky enough that we are not experiencing this/or it possible in the coming days.