Are the Bostitch gas cordless nailers available in a bump fire mode?

No, only sequential cycle is available. This sequence is critical to activating the internal fan that evenly distributes the gas in within the combustion chamber in preparation for the combustion stage, and then allows enough time between cycles for the fan to exhaust spent gases / heat and to pull cooling oxygen in from the atmosphere in preparation for the next cycle. Also, the excessive speed that bump fire would allow would cause the tool to overheat and malfunction.