By AMD's own admission in their E3 slides (see below), it's going to be "comparable" to Intel at gaming, but it won't always win. In fact, looking at the slides, and judging by AMD's down to earth stance with Zen since its launch, it's expected that Zen 2 will at best still lose by a little overall against the 9900K (vs the 3900X), with 3 wins, one tie and 4 losses.
It will be interesting to see how the reviews go about comparing these head to head, price point to price point, because it does seem like that extra cache and higher boost clock on the 3900X does make a difference against the 3800X and may indeed be the best gaming CPU from AMD, even more so now that Windows will have a topology awareness scheduler, which I'm assuming will make the biggest difference with the 3900X's two CPU chiplet configuration. From the slides, the 3900X never loses against the 3800X and then goes on to win in 3 out of 8 games.
In the 3900X vs the 9900K slide you can see that Intel wins 4 games (in two of them by a very small margin), AMD wins 3 games and they tie in one. Interestingly enough, the results of which CPU wins in each game varies by slide. The 3600X against the 9600K it goes one way in some games, and then in the other slides (3800X vs 9700K and 3900X vs 9900K), it goes other ways. But overall though, by AMD's own admission, Intel generally will win in the majority of games (5 out of 8 for the 9600K and 9700K and 4 out of 8 and one tie for the 9900K), but overall it's going to be close and in some games the difference will be really small.