The optimal way is to play 1.Nf3 Nf6 2.c4 g6 and then 3.Nc3 or 3.g3.  Few players find the only way here to go normal g3-Grynfeld positions.

This has never stopped me from playing d5. Would other players, who play Grünfeld against the fianchetto, be stopped? I suspect not, as they certainly will understand the difference between playing d6 vs. d5 quite well.

Maybe I am a bit opinionated because my only win against the Fianchetto KID when I did not play the Symmetrical ...c6 lines was out of maybe 7 or so games in this variation involved sacrificing my entire queenside in the variation with ...a6/...b5/...c5 and a subsequent kingsideattack where I was probably busted more than once. The others I simply was positionally squeezed and without counterplay.

Likewise, I play the Fianchetto Grünfeld because I play the Grünfeld now, and I have so far never lost in serious games, but as White when I play the Fianchetto vs. Grünfeld I usually get plastered badly, and lose even worse and more often than against other defences. I would be lucky to remain =+ and not worse than that by move 15 in any Fianchetto Grünfeld line with White. If I were KID player still, I would personally, now knowing what I know, transpose into the g3 Grünfeld systems. My instinct is that KID players would probably try to remain principled and play ...d6, but there are always those who are more flexible and try to maximalise the position. 

I do not have Janjgava's book but somehow it feels to me that after the 2010 Olympiad in Khanty-Mansisyk, the Fianchetto Grünfeld is still up to now in very good shape, much more than it ever was before, which might tempt a KID player to play ...d5.

I like to play the Fianchetto KID, but obviously the direct move order 1. d4 Nf6 2. c4 g6 3. g3 just allows the Fianchetto Grünfeld with either the knight or the ...c6 Symmetrical set-up. I know that 1. c4 and doing some sort of dance can lead to the Fianchetto KID by transposition, but in a practical repertoire this is actually quite cumbersome to work just to avoid the Fianchetto Grünfeld. There is also the big problem that 1. c4, although Black can be a KID player, can just play the main lines 1...c5 or 1...e5, which causes even more work for a mainly 1. d4 player.

I had another, more straightforward idea. Do any players avoid the Fianchetto Grünfeld but play 3. g3 to do Fianchetto vs. KID by bluffing, i.e. playing it hoping that Black plays ...d6 instead of any ...d5? Obviously just playing it blind with no information about an opponent might be risky, but if for example, men ser in the database that the opponent plays the KID, would it be a good gamble to play 3. g3? 

I am not sure how many KID players transpose to the Fianchetto Grünfeld with 1. d4 Nf6 2. c4 g6 3. g3 Bg7 4. Bg2 d5 or 3...c6. As an former KID player, I played the Symmetrical Fianchetto Grünfeld, but that was just because I had no idea what I was doing generally against the Fianchetto set-up. My instinct tells me that KID players might just not sense the bluff and play the ...d6 set-up, where my personal feeling is that the entire Fianchetto KID is just +/=, but the Fianchett Grünfeld being = in both ...c6 and ...Nxd5 lines. Is this a good idea against someone only if men know that opponent plays KID (and not Grünfeld?)