What do you mean by "is it worth it"? What exactly are you looking for in the game? If you're after a simulator on par with GT5, you're going to be disappointed.
As for Gymkhana, it is both free-roam and competitive. After completing certain parts of the game, you can enter the Battersea Compound. Completing more of the game opens up other sections, and by completing certain objectives in the Compound, you will eventually be able to access everything at once.
It's also competitive. There are eight other gymkhana arenas; four in (and around) the LA Coliseum, and four on the Monaco waterfront. You then get ninety seconds to drive around and perform as many tricks as you can, which you are scored for. These arenas are smaller than any of the three Battersea zones, and there are no competitive events in Battersea during the Dirt Tour (though it is used for three of the four party mode games).