Attaching files to an email is generally pretty simple in Gmail. When creating an email, you can look for the little paperclip in the bottom of the composition window and click that. This should open the File Explorer so that you can select a file to send with the email. You can freely add multiple files by repeating this process (a shortcut for this would be pressing Ctrl and clicking on multiple files to select multiple files to add to the email at once).

If you're on a mobile device and would like to send an file through Gmail, you should find a similar option at the top of the composition window.