When you create a Governor space, a dedicated wallet is automatically generated to sponsor gas fees for governance interactions.
You can find the address of this dedicated wallet by navigating to Settings > Authenticators in your space:
To activate gasless voting, simply top up this wallet with ETH (or the relevant gas token for your network). There is no fee charged by Snapshot for this service. Keep in mind that if the wallet balance runs out, the gasless functionality will be disabled.
Once funded, voters will be able to cast their votes by simply signing a message instead of submitting an onchain transaction. The signed message is then relayed onchain to the Governor contract by Snapshot's meta-transaction relayer, which submits the vote on behalf of the user and covers the gas cost.