Yep, I've got the KJW m700 and the modelworks m40 kit.Can't remember who used to make the m40a3 gas version (pretty sure there was one though), but the m700 based one is expensive, since you have to get the m700 then also get the conversion kit, but once it's together you get the m700 system behind it and the Action Army m700 hop unit is probably the best hop unit I've ever worked with.
The only other gas m40 i know of is the VFC but that's an a5 barrel. But there might be some like 1990's build that there's only 9 of in existence.