There are instances where Mail Sync is not connecting properly through Gmail. This is a known issue, and it is being addressed.
This does not mean that Mail Sync cannot connect to a Google account, however. In fact, you can use your Google account to serve as the main connection mechanism.
In this case, you will need to generate an application-specific password in your Google account settings. This is a special password that you will need to use to connect Google with SharpSpring.
For more information on connecting Mail Sync via a Google account, refer to Enabling and Configuring Mail Sync for Google Accounts.