After a while, I stumbled upon this post and this other post, stating that openconnect client can connect to GP vpn.
To install openconnect is fairly easy. Just fire up your terminal, and use below command to install openconnect client
$ sudo apt install openconnect -y
Once installed, you just have to use below command to connect to your GP vpn
$ sudo openconnect --protocol gp -u foo vpn.server.com
You will get some warning about "Certificate failed verification", just answer yes
Certificate from VPN server "vpn.server.com" failed verification.Reason: signer not foundTo trust this server in future, perhaps add this to your command line:--servercert pin-sha256:1YWmjjGL3wppl245dRc3/p+mytteBnvaVz456DQY+wutt=Enter 'yes' to accept, 'no' to abort; anything else to view: yes
It will later ask for you password, just put in your password
Connected to HTTPS on vpn.server.comEnter login credentialsPassword:
You will know that you are connected if you find something resembles below line
Connected as 192.168.100.72, using SSL, with ESP in progress
Try to access your internal server, and you should be able to.
Rejoice!