First you have to go and add SMTP Mail Server configuration in your Jira Server. It should look like following, add your gmail address in From address and Email prefix.
After that if your Test connection is not working, follow these steps further.
Important: To create an app password, you need 2-Step Verification on your Google Account.
If you use 2-Step-Verification and get a "password incorrect" error when you sign in, you can try to use an app password.
Go to your Google Account.
Select Security.
Under "Signing in to Google," select 2-Step Verification.
At the bottom of the page, select App passwords.
Enter a name that helps you remember where you’ll use the app password.
Select Generate.
To enter the app password, follow the instructions on your screen. The app password is the 16-character code that generates on your device.
Select Done.
Create use app passwords and create unique 16 digit pass that you will copy and paste to your outgoing mail in data Centre settings :
Go to your Email configuration in Jira and now use and copy generated 16-character password in password like below.
Now if you make test connection it should be successful !
Now you should be able to send an email to gmail account using for example Power Action application or any other apps for example that are sending emails inside Jira.
These are proper settings for SMTP Mail Server configuration :
