JudasIscariot: Any little bit helps :) Not asking you to go over everything with a fine-tooth comb just if you see anything glaringly obvious :)
Yep, I found a way to make the process go a little faster which I'll describe here so that others can go over their libraries without it being as difficult as it seems at first. I sent you a list, thanks again.
For those who want to check their game catalogue against the store catalogue genre tags, do the following:
1) Open up a browser tab for the store game catalogue:
http://www.gog.com/games 2) Configure the page to: OWNED: shown, ORDER: by title
3) Open a separate browser tab to the right of that and load up your library:
https://www.gog.com/account 4) Configure the page to: SORT BY: title
5) Open a 3rd browser tab with your library loaded sorted by title with no genre filter applied.
6) Switch to the store page tab and select the genre you want from the genre dropdown, then switch to the library tab and do the same.
7) Scroll through the store page listing looking for games that are greyed out and flagged as "owned", make sure you configured this in step 2 properly.
8) For each game you find owned in the store page, switch to your library view and see if it shows up with the genre filter applied. For games that are multiple games in a bundle such as Might and Magic 1-6 there should be multiple entries on your unbundled game shelf. If any of them are missing at all, or any of the bonus games, expansion packs etc. that expand from the unbundling of games in the library shelf, make a list of them. You can use the 3rd browser tab that shows your unfiltered library to corelate unbundled games with the other two tabs if need be.
I sent Judas a list of the RPGs missing in my library view filtered with genre set to role playing but that'll only notice games I actually own which are missing the tags, so others might want to consider using my above method to detect mis-tagged games in various genres through their own catalogues. It'd be even better if GOG could automate this though. :)