There is currently a free weekend on Steam for the game, therefore it is available in your Steam library even if you don't own the game.
There is also a glitch enabling you to get the game for free, but first I have to warn you that this is a Steam glitch and once fixed Steam may revoke the game from your library or worst case scenario ban your account for exploiting it
1. Find 'Dawn of War GOTY' in your library & click on it
2. On the right hand side under 'Links' click on 'CD Key'
3. Click 'Copy Key to Clipboard'
4. Activate the key on Steam
Note: It also seems to work with Dawn of War II & Chaos Rising