You need 2-step verification on your to use an Application Password for the '''googlemailGmail''' account. [https://support.google.com/mail/answer/1173270?hl=en Here google explains]
how to create an application specific password which can be used with the booking system. If you try to use your normal googlemail Gmail password Google might block the emails from sending and treat them as spam.
Google accounts which have a menu "My Account" >> "Connected apps & sites" may need to set the setting "Allow less secure apps" to ON to allow email sending via gmail