If you mean that you want to use the SCAN button on your Brother HL-L2390DW to "Scan to Email Attachment" as described in your User Guide here (see below), you can't do that with Gmail because, as explained above, you can't make Gmail your default email client.
The Scan to Email feature does not support Webmail services.Use the Scan to Image or Scan to File feature to scan a document or a picture, and then attach the scanned file to an email message.
The machine scans to your default email client.