I am not sure if that would violate the terms and conditions of using AWS Storage Gateway VM, what I know is that the default sguser is pretty restrictive and not much troubleshoot could be done from that user (well, I kept seeing activation failure when tried to activate the Storage Gateway service and it looks like port 80 was rejecting the requests for some reasons).
So, as a last resort I tried to "break in" the VM by booting it into single user mode so that I could reset the password there (thanks God, as long as it is still a general Linux).
To reset the password, hit "e" at the Grub menu, use the up-down cursor to scroll to the line start with "Kernel". Once you are on that line, press "e" and then append "boot single" to the end of the line and press enter and "b" to boot. After that the VM will be booted to single user mode and you could simply type "password root" to reset the root password.
So what makes it good to reset the root password? You could get into this VM and do further troubleshooting. In my case, I get into the AWS Storage Gateway VM and found that port 80 didnt come up at all for some reason so that I could check on something else.(though, with further investigation, it looks like i didnt configure proper ntp options for the VM and ESX host)