Completely deleting an XRP Ledger wallet is possible with the Deletable Account Amendment from May 8, 2020, however, GateHub does not support this option yet.
Until then, deleting an XRP Ledger wallet from your GateHub account will not make the XRP reserve available.
Please note that your XRP wallet is not tied to your GateHub account. You can import your XRP Ledger wallet to a different service that already supports the previously mentioned amendment with the use of the wallet's secret key.
Or you can store your wallet's secret key in order to restore your wallet at any time (for example in the event when deleting a wallet and obtaining the last 20 XRP becomes more broadly available), even if you close your GateHub account now.