You could, I guess, compare maps for your own use (that's their purpose, after all); but what you must not do is transfer knowledge from Google's map into OSM (or vice versa).
Would "I looked at GMap/OSM comparison and then reused what I saw in OSM" be wrong in the context of law? Would that be considered copying from Google? IANAL, but it seems to me that Google's position would be stronger than yours - they do have a "no derivatives without written permission" clause in the ToS, and an automated check looks as a completely different intent than normal map use.
I'm not clear what you mean by "reused what I saw in OSM". You probably can get by with comparing google maps to OSM, but if there is any difference you need to physically go to the error location yourself and see what is really there.
It is common for map makers to intentionally put a few errors on their maps, if those errors appear in someone else's map that is proof of a copyright violation. The location and names of roads is a fact that cannot by copyrighted, but errors are a creative work that is subject to copyright. I first heard about this in the 1980's when map meant paper, I suspect google is doing the same thing in some form.
When contributing to OSM, it's best to not use any other maps while you work. Consider it a form of clean-room mapping, if you will. Editors like iD and JOSM already provide powerful access to approved imagery sources and GPS traces. There's no need to 'steal' information from commercial maps like Google.
I wasn't suggesting to automatically copy infos from gmap to osm but to provide something like a notification that the two maps differ in a place/road.
So, i understand that i can't create an automated list that lists what are the differences between gmaps and osm.
I would advise against comparing, too. A good lawyer could prove that diffs between the maps are made from Google's content. In short: unless it's explicitly whitelisted (e.g. Bing Aerial is on the whitelist, Bing Maps is not), avoid.
Would "I looked at GMap/OSM comparison and then reused what I saw in OSM" be wrong in the context of law? Would that be considered copying from Google? IANAL, but it seems to me that Google's position would be stronger than yours - they do have a "no derivatives without written permission" clause in the ToS, and an automated check looks as a completely different intent than normal map use.