Something isn't right with your thread.
You don't HAVE
GMAIL on your laptop. It's a cloud computing email service that by its strictest definition online and online only. You need internet access to send and receive email. The Desktop notification isn't an email program in itself.
To send an attachment, sign onto your Gmail account via your internet browser and then compose an email. Attach the document with the Attach a file
button. It will open a box for you to choose your desired file to upload.