Will Smartthings ever support ZGP "friends of Hue" devices?

I think it unlikely that smartthings will add the green power commands to its own Zigbee stack, but I don’t think it matters as long as you are willing to use a hue hub to bridge them.

There is already a community-created edge driver that can bring in a number of these devices. This should be the most feature-rich integration. But this does require that you have a SmartThings/Aeotec hub.

[ST Edge] Philips Hue LAN [BETA]

Alexa can also now use many of them to trigger Alexa routines, which can give you some partial integration that way. (You still need the hue Bridge, but that would be an option for people who don’t have a smartthings/Aeotec hub.) I myself am already using this method with a few green power devices, including a friends of hue switch.

Here is a screen from the Alexa app (not the smartthings app) for creating an Alexa routine (not a SmartThings routine) using a Friends of Hue switch.

Finally, once both parties have full matter support, there may be a possible matter integration as well. If so, that would likely bring in more devices than the current official integration.

So you can use them now, it’s just not a full integration, and I don’t expect that there ever will be, unless you count matter as that. But it’s a lot better than it was when they were first introduced.