Shimano’s GRX gravel groupset

Recent Gravel tests have pitched 1 x 13 & 2 X10 as best gear ranges ...to be honest if you are going to serious gravel and green lane/bridleways...go for the 10 speed for robustness and cost effectiveness rather than "I've got more gears than you" . Serious gravel riders put gear ratios ahead of speed. I built my own bike as I'm in the trade and geared to 28mph on the road at 80 rpm cadence...and 6mph at the same on the sandy and tough stuff loaded with luggage...2 X 10 SRAM mullet build. As an example too....built up out of the box just yesterday a 2 X 10 grx Topstone... Customer is an aero bike road jockey, brought the topstone as a winter bike. .... whatever route you go....Sade riding

I got the 2x11 600 shifters and wish I'd got the 800. What I didn't find out until after buying, in spite of reading multiple reviews and guides to the grx range, is that only the top end levers have the ability to adjust the brake lever throw/pad position. That is priceless when it comes to set up annoyance, if you're a diy mechanic the time saved from unbearable tinkering will be priceless.

FWIW i'd pay a premium for 11sp GRX600 but not GRX800 mechanical as the only difference is servowave and 12g. 11 speed also lets me put road wheelsets on the gravel bike.

The best part of GRX is the moved lever pivot position that makes braking from the Hoods almost 1 finger light and afaik they all have that. 

I have GRX815 DI2, and whilst I've never ridden it back to back with 600 levers I can't say the servowave is noticable compared to my 105 and Ultegra disc brakes, the hood braking is a revelation though.

GRX 800 DI2 is probably worth it for the more "hooky" hood tops if you are into serious gravel but obviously you have to be bought into the electronic option in the first place.  I'd like to see those filter down in GRX v2.

If the only difference between the bikes you're looking at is the groupset, then I would tend to agree with the other comments. The "feel" will be very similar between the 400, 600 and 800. One thing that would be different is the 800 levers have "servo-wave" braking - but I can't comment on whether that makes any appreciable difference. Obviously there's a weight difference (which adds up to ~170g across all components) and the extra gear, but I don't think either of those would be huge issues.

One thing that may be worth considering is availability of compatible parts and/or cross-compatibility with any other bikes you have in your household. Generally speaking there's a lot of cross-compatibility between current 11-speed groupsets (105, ultegra, dura-ace, grx-600 and grx-800) while in 10-speed there' only grx-400 and tiagra. There is some compatibility between 10- and 11- speed systems but much more restrictive. So if you have several other 11-speed bikes in your household, then it might be worth considering sticking with 11-speed. And if current supply chain issues carry on, I would be inclined to say that 11-speed parts pop up more regularly on eBay than suitable 10-speed parts so that's worth bearing in mind too. 

I would say not. It's basically the difference between Ultegra and Tiagra. You're paying a premium for 2 extra gears and lighter components, that's it. Shift feel will be similar. And, if you actually used it as a gravel bike components will wear out quicker and you might bash a mech or a chainring, and the parts are consiberably cheaper for the 400 series. Unless you're chasing every gram in weight saving or want the best, stick with 400/600.