On the source chain, you need gas tokens (e.g., ETH, MATIC, etc.) to pay for:
Swapping your source token into XTZ/WXTZ.
Bridging the XTZ/WXTZ over to Etherlink.
On the destination chain (Etherlink), you also need gas tokens (XTZ) to pay for:
Executing the second transaction that swaps your bridged XTZ/WXTZ into USDC.
Any future transactions you might perform on Etherlink (e.g., swapping again, sending tokens, interacting with dApps).
Without gas tokens on both chains, your transactions cannot be processed, and you risk having funds stuck in an intermediate state (e.g., only bridged as XTZ/WXTZ but not swapped into USDC).