There is at least one possibility to exchange BitBTC and BTC in the DEX, without any need of Gateway or third parties.
Until the Atomic Swaps begin to work, I have made a proposal that I think should be taken into consideration:
For example: Alice, Bob and the witness create a 2-of-3 multisign BTS address. Two signatures are required, the witness and Alice or Bob to withdraw the deposit.
A has 0.1 BTC and B has 0.1 BitBTC and both want to exchange them. For this, A and B deposit an escrow of 0.12 BitBTC each in the BTS multisign address. A and B report their bitcoin addresses to the DEX and both close the operation. The DEX checks periodically if the bitcoin has been transferred between the specified addresses and if so and there is a confirmation, he signs and returns the escrow to A. It also checks if BitBTC has been transferred. In that case he signs and returns the escrow to B and the operation ends completely.
It is necessary to have BitBTC to buy BitBTC, but as it is possible to iterate the procedure, a very small amount is enough. The Dex could handle this iteration, performing the interchange in stages.