I knew it'd happen eventually. Not too surprising it was G&P either since they've been probably the biggest player so far in terms of other accessories for the TM 870.
Kills a fair few birds with birds with one stone to my mind. They already make rifles designed for propane so this shouldn't struggle with the pressure like the Marui tanks do with makes their 870 a product I could actually buy (at last) as well as adding the AR accessory compatibility I know many have been after (myself included). Looks like you can charge gas straight in from that valve in the top rather than having to take off the stock each time, which wouldn't be much of a drama with AR collapsibles but it does make life that bit easier.
Since the inner gas tank is removable as the outer buffer tube screws in the RS way I think it should mount AR lower base plates for some nice 1-pt sling options (on top of the QD sockets already integrated) and I'd wager RS/GBBr type pistol grips will fit as well which combined with the stock options makes for a lot ton of wiggle room to give it exactly the ergonomics the end user desires based on a whole range of products already on the market. Adjustable LOP and grip angle (decided by the grip fitted) sounds about 1 million times better than the fixed stocks to me personally.
Only potential I see is that there looks like a lot of different points where components meet with o-rings to transport and store the gas, so hopefully they've got good tolerances and quality rubber parts on those areas to keep things non-hissy.
Also an old-school lighted forend for the RIS-adverse that still want to be a tiny bit tactical: