lawl. windows isn't a free o.s., neither is macosx. windows comes with some free games like minesweeper, chess, freecell (and some card games) and a few others. some linux distros come with free games. but i don't know much about them. there are free games available on windows and linux, but many more in windows platform.
windows has much more options for playing free games, or buying games on sale at very low price.
windows 7 (home premium or pro or ultimate 64bit) and windows 8.1 or 8.1 pro 64bit are best options for gaming on desktop/laptop pcs.