If you purchase a NFT and it doesn't appear in your Metamask wallet after some time, you can add it manually. All you need is the collection contract address and the token ID.
Please make sure you are connected to the right network when adding NFTs to your Metamask (Example: If you want to add an NFT that was bought on Binance Smart Chain, make sure you're connected to BSC).
Open your Metamask and go to the NFT section. At the bottom, click on "Import NFTs". Input the Collection Address and your Token ID and then press Import.
You can find the collection address and Token ID on the NFT page on Galler.io under the "Detail" section"