Had to do a run to HK on Friday. Needed to get downtown to sign some papers, so didn't take the usual airport shuttle.
DG bus station ticket window girl took the green card without blinking. Security for bus waiting area swiped it on something and got the correct tone of "Beep" to pass me through.
At the Chinese exit point, the immigration girl said something to my wife about green cards. It turns out that these can be used as part of an ePass application. Happily, the HK ePass I got for flying in and out a lot worked on the HK side.
On the way back in, the immigration clerk didn't even blink when I gave her the green card. We asked where to apply for the ePass. We weren't sure if this was done at the border or back at the DG Naughty Foreigner Containment office. Turns out it was at the border.
The clerk asked if I want use my existing Res permit (expires in March 2019) or the green card. The green card was the logical choice and turned out to be a very wise move. My passport is less than 6 months old and its expiration date was used since it was earlier than the green card. If I had chosen the res permit, the ePass would have expired in March 2019.
So, unless you've got a very long term res permit or visa that qualifies (I have no idea if tourist visas can qualify for Chinese ePasses or not), a green card can get you an ePass that runs until either the card or the passport expires, whichever comes first.
I also was not sure if this was for HK crossings only, so asked. I was told it's for all entry/exit points.
At the Lowu/Lohu bus station, they sell tickets alongside the bus itself with a portable ID reader. The ticket girl couldn't figure out how to enter a green card, so my wife had to help her find the correct category.