Hi @Lili
Check out this help article about using files in Zaps: https://zapier.com/help/create/basics/send-files-in-zaps
Just came across this and wanted to clarify that it looks as though an ID number rather than the actual file had been selected in the Attachments field on the Gmail step.
As per the guide Troy linked to, in order to send a file, either a file object or publicly-accessible link to the file needs to be selected.
Depending on the access permissions for the folder the file is added to, you may also need to add a Add File Sharing Preference Google Drive action. This would action allow you to create a sharable link that you could then select in the Gmail step as the publicly-accessible link to the file. :)