If not, it is possible to purchase older versions of DOS on eBay. GWBASIC can also be downloaded for free from a few websites, such as the site linked to at the end of this page. A few sites also offer versions of DOS for download which include GW-BASIC.
The process to actually install GWBASIC is simple; if it is in a ZIP file, use WinZip or PKUnzip to decompress it, then copy it to the directory it will be used from, and run it. It should work in any version of Windows or MSDOS, although you can only use its default screen mode in XP.