Polygon transactions on OpenSea are GAS FREE.
Wallets such as MetaMask generally don't come set up for the Polygon network, you have to do it manually. See guides below.
For other wallets, please use a search engine (Google) to find instructions.
If you don't have a wallet, you can get one here: https://metamask.io/
There are several ways you can top up your Metamask wallet with Ethereum.
The cheapest method and the most common is to use an exchange (such as Binance) to send MATIC to your wallet, then exchange it for ETH in the wallet using the swap function. Polygon fees are tiny so the transfer & the swap will cost almost nothing. You may already have currencies in an exchange that you can exchange for MATIC or you can send your local currency (FIAT) to the exchange using a bank, debit or credit card (depending on the exchange used) and buy MATIC with minimal fees.
If you already have ETH in your Metamask wallet (Ethereum Mainnet) and you want to use that, then there is no escaping the Ethereum gas fees. No matter if you send to an exchange first, or use one of the method's linked below, you will have to pay gas.