As far as I'm aware, they are like a slightly different format that GT5 recognizes but 6 doesn't. They only ported the Top Gear track, but it's a bit bugged, there's no shadows cast by you car I think.
Shame, as if it weren't for them and the Used Car Dealership mod for 6 not being out yet, I would not have any need for this game and would be able to free up some space on my PS3's hard drive. There's also the events I guess, like the special events section, and the endurances, but I'd rather play scaled-up versions of the ones in GT6, maybe up to 6 hours for the 24h ones.
There's also the course maker, which might be nice if it's possible to add custom tracks to custom events. More rally events would be nice. Better than the only rally event in 6.