Would it ever be possible to see the actual commands used in the gimp undo history instead of just “G’MIC-Qt”, or is it just not programmatically possible?
I realize G’Mic will never be baked into gimp, but anything to tighten integration would be fantastic. I really can’t think of using one without the other.
Not possible. The only way I can think of is to set verbosity to layer.
Apparently, this is not possible. What appears in this Undo History seems to be the blurb name, which is a name used to register the plug-in to the GIMP API. This name is set at the very beginning (so even before the user choose a filter).
This option has disappeared recently.
Deprecated, withdrawn or bug? I know it doesn’t work in certain hosts…
Deprecated. This option was mainly useful to copy/paste the G’MIC command line from the layer name, but a new button has been added specifically for that purpose, which is more practical to use.
And having a different behavior in the way the plug-in names an output layer, depending on an plug-in option, was not a good idea anyway.
As @Jehan is changing the plugin API at the moment for GIMP 3, perhaps it’s a good idea to a add a “set_Undo_Name” function.
Having an optional “label” option would be an interesting idea? Like gimp_image_undo_group_end (image, label)? Then the undo title would be “blurb: label”, for instance: “G’MIC-Qt: Drop Water” (or whatever the plug-in dev find a good label to describe what the plug-in did)?
I don’t understand which feature you are talking about here, but from the next message from @David_Tschumperle, I understand the feature to be removed was a good thing and it has been advantageously replaced by something better… (right?)
Just a note: sorry for plug-in devs, I am breaking quite a bit the unstable API these days, but I figure it’s better to do it now than be stuck for limited feature for the next 10 or 20 years. 
@David_Tschumperle refers to the option of copying command from the gui filter. As a Krita user, it is great to have that. I can finally do the verbosity to layer, but by hand. There is still the need to create new layer on gmic in Krita.
That could be nice indeed. Not sure if this is really needed for other plug-in though.