A family of Microsoft word processing software products for creating web, email, and print documents.
I guess you are using one of the commands in Word to send your file directly as an e-mail attachment? Note that when you do that, Word will start your default e-mail program, which may be programs such as Outlook, Thunderbird, etc. If you don't want to use any of those programs, just save your Word file to your hard drive, and attach the file to an e-mail from within Gmail.