Usually when a move in a relatively positional setup such as this is omitted, it doesn't mean that it's a clear mistake. It only means that the author doesn't view the move as a critical move that challenges the opponent's setup as much as other moves. Therefore, it may not be the move to prioritize, as other moves can be more challenging.
It could also be that the author doesn't know really what the plans are, and tries to make him/herself look more certain about their topic than they are.
One question though, which needs to be answered by you: Does this move give you many difficulties in your games? If that is the case, you will be forced to look up some statistics and see if the move is a legitimately challenging line, or if you just don't know how to play against it. In this case, you will need to study games played in the variation and learn important ideas by yourself if you want to be more successful. It may be a difficult thing to do, but sometimes chess isn't very easy.
I've taken the liberty to consult my personal database, which for the moment lacks some games due to my negligence with database maintenance using TWIC these last weeks, and I found this:
6.Ng3 is likely not a bad move. It has a 55% score, with roughly 600 games played in the line. The highest rated player who played this had a rating of 2535. The move is not to be scoffed at, if you face it a lot.
HOWEVER
There are better moves in the position:
The main line, 6.Nxf6+, has a 63% score in roughly 4400 games, with the highest rated player having a rating of 2838;
The second most popular alternative, 6.Bd3, has a 62.5% score in roughly 1800 games, with the highest rated player having a rating of 2851;
The third most common alternative, 6.Bg5, has a 57% score in roughly 1600 games, with the highest rated player having a rating of 2851.
The top rating is due to Kasparov in all the lines, but all the top players in the world have used at least one of these three main moves when faced with this position.
