You can make the GOP area dragable using dynamic patching (see attachment). I'm using iemguts and cyclone here, although you can find pure Vanilla alternatives if you need them.
I take it that nobody knows a way of sensing the GOP resizing, as I had originally asked? I suspect it might be impossible (although I've achieved lots of "impossible" things on PD in the past).
Date: Mon, 23 May 2016 16:30:50 -0700
Subject: Re: [PD] sensing GOP resize
From: x37v.alex@gmail.com
To: liamg_uw@hotmail.com
CC: pd-list@lists.iem.at
oh huh, wild.. so you still can't use the mouse to change the bounds of a GOP in the GOP patch itself but if you create an object for that GOP you can resize that then go in and save and get the resized GOP area.. would still really love to have mouse draggable bounds inside the GOP abstraction.