Gas Fees are transaction fees that crop up in many Blockchain events including minting NFTs.  The official explanation is that the people running all the computers storing and validating Blockchain data expect to be paid for their trouble.  This income will partly be used to fund the power used by the Blockchain servers and explains why server farms are more profitably located where electricity is relatively cheap.  Payment is not guaranteed but depends on being the first server to collate transactions into a block and having that block certified and verified by other parts of the network. Having more and faster servers will increase the likelihood of the Blockchain paying out.

Gas fees are not fixed.  They will be cheaper when the Blockchain is less busy.  For Ethereum this tends to be in the morning in the UK.  There are only small savings between weekend and weekday fees.  The fees are paid in cryptocurrency so also depend on the relative value of that currency.  It may be some consolation that a cryptocurrency trading at a low exchange rate will mean lower fees and cheaper NFT base prices.

An NFT only needs to be minted once.  The process guarantees that it is unique and allows it to be sold on the Blockchain.  There are likely to be other trading fees from buying and selling NFTs.  These depend on the platform used for trading but like an auction house there will probably be fees to the buyer and possibly the seller as well.  On OpenSea for example the base fee to a buyer is 2.5% but the original NFT creator may demand an additional creator fee of up to 10% of the transaction each time that NFT is sold.

Some cryptocurrencies avoid Gas Fees altogether.  OpenSea allows minting through Ethereum (with Gas Fees) and Polygon (no Gas Fees).  The process is broadly the same for working with Ethereum as with Polygon so why not only use Polygon?

The problem with Polygon and other less popular Blockchains is that they are less popular (a smaller potential market) and less robust (fewer nodes).  To trade in NFTs requires a wallet of the appropriate cryptocurrency.  Funds can be withdrawn from either or transferred directly between the two.    Unfortunately Gas Fees will be invoked in all these cases.  Any buying and selling of an NFT will be in the related cryptocurrency so the potential value of that NFT depends on the strength of that cryptocurrency.  NFTs can be moved between wallets (either your own or to give the NFT away) again with potential Gas Fees.  Moving to another cryptocurrency ecosystem would require minting the NFT again and it would no longer be fungible.