How i can trade bitcoin on next client? deposit with multigateway, and than is an asset for trading, or i must create them?

Essentially when you deposit BTC to the Multigateway it will be sent to you as the mgwBTC asset. You can trade that on the NXT asset exchange... but only BTC/NXT. If you want to convert it back to BTC you can use the multi-gateway again.
However, if you want to convert BTC to say DOGE, you can sell your BTC assets for NXT, then buy DOGE assets. Then if you want you can use the mgw to withdraw the Doge to your Doge wallet.
So, basically the NXT AE with the addition of the mgw becomes an exchange where you can trade any supported mgw coin with NXT rather then BTC that you would use at say BTER.