I haven't followed the Gwen story, so I'm learning about her as a person for the first time in this thread. I gather a lot of people don't much like her because of some of the things she says. Fine. Maybe I wouldn't like her either.
On the topic of her marathon, though, here I have a better formed opinion. The marathon is a cruel mistress, and on race day absolutely everything has to go right in order to race to your capability, and your ultimate capability that day depends on your background (time in the sport, aerobic development, …) and the effectiveness of the marathon build up. If any single thing goes wrong, the day goes to $hit, and this can happen to anyone, novice to elite (although is maybe less likely to happen to an experienced elite runner).
My read of the info in this thread is that there's a better than 50-50 chance she just had a bad day (the positive split, off what should by any reasonable interpretation be a safe pace, confirms this to a large degree). There's no way to guess at real potential when somebody falls apart in the marathon.
My thinking is that, absent of strong evidence to the contrary (i.e. an even split 2:35 in good race conditions), she is most likely a sub-2:30 marathoner. I also think there's no good reason to think she can't improve on that with experience.
First thing she needs to do is shake it off. Second guessing and negativity generally isn't a productive path forward.