Google Compute Engine SSH连接超时（错误码255）求助

我最近尝试通过SSH连接我的Google Compute Engine实例，执行了以下命令：

gcloud compute ssh username@instance --ssh-flag="-v" --zone=us-east1-b

结果返回了超时错误，完整的调试日志如下：

OpenSSH_7.2p2 Ubuntu-4ubuntu2.4, OpenSSL 1.0.2g 1 Mar 2016
debug1: Reading configuration data /home/username/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: Connecting to ip-address [ip-address] port 22.
debug1: connect to address ip-address port 22: Connection timed out
ssh: connect to host ip-address port 22: Connection timed out
ERROR: (gcloud.compute.ssh) [/usr/bin/ssh] exited with return code [255]

（注：上述日志中的ip-address均为实例的实际公网IP地址）

我怀疑是实例所在的网络拦截了当前连接，于是又用telnet测试端口22：

telnet instance-ip-address 22

得到的结果同样是超时：

Trying instance-ip-address..
telnet: Unable to connect to remote host: Connection timed out

有没有大佬能帮忙分析下这个问题的原因，以及给出解决办法呀？

内容的提问来源于stack exchange，提问作者sandeep cb