Found 2 exchanges where Gas is traded for BTC. According to the data on 27 August the most optimal price to exchange GAS for BTC is ₿0.000015 for 1 Gas. This rate is fixed at CoinEx at the maximum trading volume of the pair GAS/BTC - 0 BTC.
The maximum daily trading volume is observed in the GAS/KRW trading pair at Upbit and reaches ₿ 88.57 (0.708% of the exchange's total trading volumes) at the price ₿ 0.000016 per 1 GAS.
The minimum price to buy GAS for BTC is 0.000015 - on the exchange CoinEx.
The maximum price to buy GAS/BTC is 0.000017 - on the exchange Upbit.
Warning: some GAS markets show an abnormally wide spread — those quotes may be unreliable (thin liquidity or off-market trades). Cross-check the volume against several venues.