The Properties Widow does not appear to show all levels of the hierarchy. The Show Tree Structure window has an additional layer that needs to be traversed to get to the TextBox() you are looking for
I was able to get to the TextBox() with the below modifications to your illistration
show(start= report(gb)["Graph Builder",AxisBox(1)]);
show(step1 = start << parent());
show(step2 = step1 << parent());
show(step3 = step2 << Prev Sib());
show(step4 = step3 << child());
show(step5 = step4 << child());
which has the below results
start = Report(gb)["Graph Builder",AxisBox(1)] = DisplayBox[AxisBox];
step1 = start << parent() = DisplayBox[GraphBuilderAxisBox];
step2 = step1 << parent() = DisplayBox[GraphBuilderComponentBox];
step3 = step2 << Prev Sib() = DisplayBox[GraphBuilderComponentBox];
step4 = step3 << child() = DisplayBox[GraphBuilderTitleBox];
step5 = step4 << child() = DisplayBox[TextEditBox];
Jim