It depends on the gateway manager and where the coins are held in reserve for users that wish to switch back to the main network. There can be multiple gateways to manage one asset such as on the Stellar Lumens network for example. A gateway manager would have to keep a large amount of coins on reserve to keep the exchange solvent. In this model the core asset functions more like a scarce commodity compared to it's current operations. It's also possible to make a custodial twitter/telegram/reddit bot that users can interact with to shift coins between different blockchains at more efficient speeds.