Certain exchanges will allow you to collect GAS by storing your NEO there. I know that Binance and KuCoin are the two exchanges that permit NEO holders to collect their GAS rewards on their platform.
Also, Neon wallet and Nano Ledger S Hardware Wallet will allow to collect Gas tokens as a reward.
While you can buy and sell NEO and convert
neo to gas on many other exchanges like Coinswitch, Binance, Changelly etc.