Well, I think the preference of PyGtk is because of Python.
Of course comparing Python and C&GObject, C&GObject loses. It's like comparing a modern train and a horse carriage.

C++ and C&GObject, not so much. They're very similar, only GObject is more advanced because it came later and learnt from the silly mistakes C++ made (no reflection, no properties (and by that, no automatic notification or monitor pattern), no reference counting, broken virtual methods, making copies all over the place, no Variant datatype, no signals, ...)
Some mistakes they share, like automatically calling the superclass constructor at the beginning of the constructor of this class (as opposed to anywhere in it).
Some mistakes are very special to C++ and nobody would ever replicate:

#include <iostream>

class A {
public:
A();
virtual void foo();
};

class B : public A {
public:
virtual void foo();
};

void A::foo() {
std::cout << "How in the world did I get here?" << std::endl;
}

void B::foo() {
std::cout << "Hello world" << std::endl;
}

A::A() {
this->foo();
}

int main() {
B b;
return 0;
}

*runs it*
"How in the world did I get here?"
*shakes head*

Some mistakes are special to GObject and nobody would ever replicate:

the virtual methods "set_property" and "get_property" should not call their ancestor method (of the superclass).
All other virtual methods should. What gives?

But as I said, with Qt and Boost, C++ is bearable.