The short answer is that there is nothing major to worry about.
longer answer is regarding less secure, google is referring to the mechanism by which the app authenticates itself to gain access to your gmail account. outlook 2016 sends your username and password over an encrypted imap connection. there is nothing inherently insecure about that approach. it is used by any other email provider.
google would prefer applications to use a different mechanism, called oauth. an oversimplified description of that mechanism is that the app uses a token to authenticate to gmail. instead of sending your username and password over the encrypted communication channel, it sends the token over the encrypted connection.
When using properly working software from a reputable company, there is essentially no difference between the two approaches. The mechanism used by Outlook is no more vulnerable to malicious attack than Google’s preferred mechanism.