GAS is the fee for transactions on blockchain platforms that support smart contracts. It acts as the fuel for the blockchain, similar to gasoline for an engine.
When you send Toncoin on the TON network, a small amount of Toncoin is charged as a fee for processing the transaction.
GAS is paid in the network's native cryptocurrency. For example, on the TON network, it is paid with Toncoin, on the TRON network, with TRX, and so on.
Even when sending a token that is not the network's native cryptocurrency, like USDT on the TON network, you still pay the GAS fee in the native cryptocurrency of the network, which in this case is Toncoin.