What's the fastest, easiest, cleanest way to kill millions of people? You'll find that "gassing" them comes in around number 80 kajillion down the list.
Just look at the gas chambers that were used to carry out capital punishments here in the US. They're giant pressure vessels with doors that look like they belong on a submarine. Exhaust fans to evacuate the gas, fume dilution fans on the roof to safely dispose of the spent gases. As you said, the safety of the prison workers was a primary focus. These facilities have nothing in common with a gang shower room with wooden barn doors.
There will be a time to act, but it's not yet. The system is thrashing about in its death throes, but it's still very dangerous. It's hyper-focused on stopping any attempt at right wing organization because the people in charge know how precarious things are.
The time to act will be when the system has broken down. When the internal security apparatus has been purged of whites and is full of lazy, low-IQ diversity hires. When a loaf of bread costs $100. When things like electricity and drivable roads aren't a given anymore. When the common man is ready to get involved. When dissidents are being rounded up. When the system is already leaning on its 3rd-teir contingency plans just to keep itself going, that's when strikes will have the most effect.
For now we have to be subtle saboteurs and "wreckers." Find ways to throw sand in the gears of the system, ways to raise their cost of operating. Find ways to encourage or accelerate the systems collapse. Or just get normie conservatives you know to stop with the Muh Flag patriotism shtick and start thinking like dissidents. All of these processes are already underway, we just need to help them along.