I think that the combination of Nf3 and Bg5 is not particularly aggressive (white is not going directly for e4 or anything like that), but on the other hand it does more or less keep black from playing the Leningrad and usually leads to lines where black plays calmly with e6, Be7, b6 etc. (which is presumably why Beim didn't cover it). In my experience one can play these positions with just common sense, there don't appear to be any immediate plans for sharp attacks or anything like that.

However something like 1.d4 f5 2.Nc3 Nf6 3.Bg5 or 1.d4 f5 2.Bg5 definitely poses immediate questions to black (e2-e4), so I wouldn't say that Bg5 is generally not challenging for black.