Well, after Lexus gave the LX the 600 moniker, despite having a "relatively" similar tune to the LS 500, I have no idea what power range the 550 moniker would put the GX in. If we look at just the 500 number in Lexus vehicles, it ranges anywhere from a 2.4L I4 turbo hybrid with 370 HP to a 5.0L V8 with 472 HP, so this is really anyone's guess
Lexus really has done some strange things with naming. I too am expecting "GX 550" to be the 348hp tune of the V35A-FTS, but your point about that being significantly less power than the LS 500's 416hp is a good one.
It feels to me like it should be:
Tundra "SR" 348hp tune of the V35A-FTS should be "400" = GX 400
LS 500's 416hp tune should be "450" = LS 450
LX 600's 409hp tune should be "500" = LX 500
V35A FTS + hybrid should be "600h" = LS 600h, LX 600h
But, alas, Lexus wants their naming to keep up with the Germans when they truly don't have the same firepower.