Most all account recovery starts with the “Forgot password?” link on the Gmail/Google sign in page.  That link leads to:  https://accounts.google.com/signin/recovery which can be used directly.  This covers lost passwords, compromised accounts, "suspicious activity" blocks and other similar cases.  It does not apply to disabled accounts for things like Terms-of-Service or being under-age.

There are over a dozen potential questions (like the account creation date) and tasks (like entering a verification code) you can use to help prove ownership of an account.  Most of the questions are based on pre-configured recovery options like an e-mail address or phone number.  If these are not available (never configured, gone out-of-date, changed by a hacker) the number of questions available may be very limited making it hard to prove ownership of the account.

The end of the process typically has you provide a contact e-mail address which is then sent a verification code.  Please note that you are only verifying that the contact address is valid, and that you have access to it.  This verification code does NOT mean you have successfully proven ownership of the lost account.  You may still receive the "Google couldn't verify..." message, or be asked additional questions.

The recovery process is more likely to succeed if done from a computer or device recently used to access the account.  If at all possible, attempt recovery using that same computer/device, giving preference to a computer/laptop over a mobile device.

In summary, if you can not recover the account using the provided process, it may be lost.  There are no other account recovery options available.