Attachments are a very important part of the Gmail experience, and that’s especially true considering the integration of Gmail with Google Drive and vice versa. All told, Gmail still has restrictions around it that can deter unsuspecting users. Gmail limits the size of attached files to 25 megabytes, any file exceeding that limit will be converted to a link to Google Drive. Large files may struggle to download properly if the internet is slow. If the Internet seems to be sufficient, but you cannot download your file, check the internal storage of your phone or SD card to see if there is enough space for the file.
Gmail is also known for its lack of support for all file types. Some, like .gz, are blocked by the app for security reasons because Gmail fails to verify if the attachment is safe to download in the first place. Some, like .exe and .cmd, cannot be included as attachments at all, so try uploading to a Google Drive folder, or find another email program if you want to send different files.