On Ethereum, asset ownership is permanent, which means once you own a card, it will never leave your wallet unless you decide to trade it. If your cards are not appearing in your Gods Unchained account, either the wallet that holds your assets is not connected to your Gods Unchained account, or you are not signed into the correct Gods Unchained account. You can view your linked wallets on the Manage Wallets page.
Once you log into your account in the Gods Unchained launcher (the window that you use to initiate matches of Gods Unchained, look at your card collection, and build decks), the launcher will look at the wallet addresses that are connected to your account, and then display what cards you have in those wallets.
Make sure that you are signed into the correct Gods Unchained account in the launcher, and then make sure that all the wallets you use to purchase cards are connected to that account. Your cards will then appear in your Gods Unchained account.
If your cards or items belong to a wallet that is not connected to your Gods Unchained account, whether that be because you have not yet connected it or you have removed that wallet from your account, those cards will not appear.
While Immutable X is in Alpha, cards can also take a few minutes to register between a trade on Immutable X and it actually appearing in the launcher.