Exchangerate.host sources its price data from financial data providers and banks, including the European Central Bank.
The off-chain price data is fetched via Exchangerate's API by the goStables' Price Oracle node which automatically feeds the current market rates directly on-chain, for the relevant gStable, to gStableManager contract. This check currently occurs every 12 hours* on Tron Mainnet. If the rates have changed the contract is updated accordingly.
As the protocol grows, more nodes will be deployed that retrieves/publishes on-chain price data from multiple real world sources, increasing accurate real world price feeds on-chain as well as strengthening overall resiliency and reliability.
In the future when enough resources are available, the price feed nodes could be spun off into a separate product to service other projects that require on-chain data from off-chain sources.
*Will be modified to shorter time intervals in future once there is greater demand and more energy resources available for cost efficient updates.