You can follow one of two methods below:

In Freshsales, 

Go to Admin Settings > Email Settings

Now add your sales email addresses to the Team Inbox.

In Gmail,

Go to Gmail > Settings (gear icon)

Under the Accounts tab > In the Send mail as section, add the email address that needs to be created as alias. 

Once the alias is verified, go to Freshsales - Click on your profile picture on top right corner > Connect Email  > Remove the integration by clicking on the trash icon and reconnect Gmail once again.

Now, try composing an email and check if the alias addresses are appearing.