Will there be any sort of fees as a user when trading/exchanging merchant tokens or GAT coins?
When no fees involved, how does Gatcoin generate profit?
I think they must have some fees.. Also part of this fees (for example 50%) must be burt - it will do more liquidity and expansive tokens.

Yes, the business model allows for revenue from 3 streams, Merchant fees on the GAT Marketplace, Brokerage fees on the GAT Exchange and engineering and maintenance fees. More information on this can be found in sect 4.3.2 of the whitepaper
https://www.gatcoin.io/wp-content/uploads/2017/09/170919v2-Whitepaper-EN.pdf