Quote:...big....snip....
So, to reiterate my question, can I reproduce the same workflow in GIMP and make it completely automatic?
Completely automatic ? I do not think so
You can use a template much the same as PS see: https://www.gimp-forum.net/Thread-T-shirt-Mock-Up
..but it is very much up to the user to turn on / off the various layers / layer groups, changing colours / logo etc.
Quote:Smart object is being transformed to match template's product shape -- here's the first tricky part which raises questions. Is it possible to recreate such complex transformation shape in GIMP and is possible to tie it to the template file, so that print takes different shapes when overlayed on different templates?
You might be better off Krita which has a usable grid deformation filter