InstantDEX is a very cool thing, but it has nothing todo with this project, well, it could be used to exchange the BTC/NXT pair of crypto-gateway even faster

The service Crypto-Gateway provides will make it possible to deposit BTC into the NXT blockchain and withdraw it. Kinda similar to multisig-gateway, but with MGW you can only deposit withdraw, you can't really trade. That is where the CG will perform best.
I thought I would clarify....
InstantDEX is peer2peer exchange-less trading using the NXT block chain to provide escrow is the easiest way (if not completely correct way) of putting it, in order for InstantDEX to work you have to let it operate on your coin wallets (I would set up wallets specifically for DEX Trading!)
MultiGateway (MG) allows transparent deposit to a multi-sig secure coin wallet linked to your NXT account, any coins deposited from your other wallets to the MG wallet are are automatically shown in your NXT account as MG escrowed coin assets on the NXT AE
If you sell the MG Asset for NXT then as you get the NXT, the MG asset is transferred to the buyer and the multi-sig escrowed coins move in the coin wallets... The buyer can then sell again or withdraw to a normal coin wallet, the multi-sig is operated by MG servers that operate on a consensus for signing the xxxcoin transactions so it only works for coins that support multi-sig - but it is completely server block chain driven i.e. no human error to creep in,,, or temptation.