SOL is the gas on Solana smart contracts. In a "normal" transaction on Solana, SOL goes towards paying the TX fee itself.

In the case of the Wormhole bridge, there are transactions on both the Ethereum mainnet side (paid for in Ethereum gwei) and the Solana network side (paid for in Solana gwei). The only lossy factor is in the bridging fees themself which are typically a "flat" fee.