I see no size limit in the spell description, which means one could conceivably use this 2nd level spell as written to push back a gargantuan creature and slow its movement toward you. Am I missing something? Has there been any kind of errata issued on this?
It is allowed by RAW, however previous versions limit its use on large creatures.
I would not allow it personally as DM to affect anything Gargantuan or larger. However the RAW do not stop it, and there has been no comments from any reputable sources. However previous versions outline what happens by size category (I believe large and large had no movement effect).
I think the width should be the limiting factor on Gust of Wind.
A line of strong wind 60 feet long and 10 feet wide blasts from you in a direction...
So based on our size chart - it should easily blow around anything 10ft or smaller (e.g. large or smaller). However Huge, Gargantuan or larger are all 15ft+. Since the gust of wind is smaller than they are I expect the intended result is to have no or little effect.
As for other reference, as far as I can see this has not been an issue addressed.