Even if I don't own an Xbox One can I purchase/download the titles for free ?
Yes. The only stipulation is that you need to have an active gold account to be able to download and play them. Otherwise, if you revert to silver, you would need to purchase them instead. Reactivating a lapsed gold account will restore access as well.
So, there's really no reason to not do it and get a decent library pre-built up. Also, don't forget to grab Super Dungeon Bros. as well, since that one's also free through the end of today, after which it will be replaced by Sleeping Dogs.