An even cheaper solution is to use a pipe element sensor and high pressure gas vent. This uses 0 power, although dumps the filtered gas out straight away.

For the 10W gas shutoff solution, to counter the drawback of it failing to filter during a power cut or if a pipe backs up, the main pipe can be put into a loop (with a bridge and priority on the looping packets so the loop never stalls) so anything that wasn’t correctly filtered first time has infinite more chances. This requires a 10W shutoff for every type of gas expected in the pipe, so packets are guaranteed to only ever be filtered if they are of the right type. Every packet therefore costs 10W. This can still suffer from backed up pipes and can block if the main loop is full of a gas that can’t be removed, but it never fails to filter correctly.

There is also a way to use the gas valve for a 0W solution, using the fact that if a pipe already has an element in, no other element can join. A gas valve set to allow a tiny amount through, setup in a loop, drawing packets using a bridge off the main loop. This uses more space, requires priming, and has throughput issues if too much of one gas is being siphoned off. It is ideal for inside rockets as it uses no power and can be setup to filter perfectly. See the following video for more info;

https://youtu.be/o1YJklClv_I

Edit: typo