I've done more than a little research on the Marshal, but it was on his life and times, not his death or effigy. I have seen pictures of what is commonly known as his effigy but I've not yet been able to view it up close (or as close as they allow).
I did not read through this with a focused eye, I just wanted to get the gist of it and from what I gather, her point could have merit given her examples. She seems to have at least a rudimentary understanding of the progression of armor, and we are talking about a matter of a few decades as Gilbert died less than 25 years after his father did; there were some advancements in armor but nothing earth shattering. That won't occur for another fifty years from then.
The knee length hauberk, coif with the donut commonly worn when wearing an early great helm and mitten gauntlets all suggest a date firmly within the Marshal's time. I wonder at the 'leather chausses' she noted on other, later effigies - or seemed like leather (maybe an early version gamboissed cuisses??), or could have been artistic license. I still haven't seen an extant example of the mailled glove with fingers, but have read a reference or two- that's an interesting project in and of itself, but I digress.
It seems like you're leaning toward debunking the author- but without being able to do as she did, I can't call it one way or the other; but if I had to just based on what I just read and what I know (which admittedly isn't much, I was and am much more interested in his life), I'd say she makes a rather convincing argument.
