Why can't gtksink work with videoconvert in this case? This seems like either a bug or some kind of API change???
Alright, I see the issue. Nevermind. The ghost pad needs to be linked at the bin level not pipeline. Whoops!
Ok, that explain. The pad_link func return an enum, was probably returning wrong hierarchy or something.
I'm sorry for the noise honestly! The error message was right but not really helpful at diagnosing the problem. I had to step back and look at my graph to really understand what the heck was going on here! Thanks Nicolas as always for the support!